Add rpc calls page (#817)

* fix: adding rpc calls page

* fix: update RPC calls documentation to include links for Polkadot.js and Subxt

* Apply suggestions from code review

Co-authored-by: Erin Shaben <[email protected]>

* Enhance RPC calls documentation by restructuring method descriptions, adding parameters, and providing clearer examples for better usability.

* Refactor RPC calls documentation to consolidate methods checking section, clarifying usage via curl and Polkadot.Js Apps.

---------

Co-authored-by: Erin Shaben <[email protected]>

Author: nhussein11

develop/toolkit/parachains/.pages

@@ -6,4 +6,5 @@ nav:
   - fork-chains
   - e2e-testing
   - 'Light Clients': light-clients.md
+  - 'RPC Calls': rpc-calls.md
   - 'Polkadot Omni Node': polkadot-omni-node.md

develop/toolkit/parachains/rpc-calls.md

@@ -0,0 +1,199 @@
+---
+title: RPC Calls to Polkadot SDK chains.
+description: Learn how to interact with Polkadot SDK-based chains using RPC calls. This guide covers essential methods and usage via curl.
+---
+
+# RPC Calls
+
+## Introduction
+
+[Remote Procedure Call](https://en.wikipedia.org/wiki/Remote_procedure_call){target=\_blank} (RPC) interfaces are the primary way to interact programmatically with Polkadot SDK-based parachains and relay chains. RPC calls allow you to query chain state, submit transactions, and monitor network health from external applications or scripts.
+
+This guide covers:
+
+- What RPC calls are and how they work in the Polkadot SDK.
+- How to make RPC calls using `curl` or similar tools.
+- The most useful and commonly used RPC methods.
+
+RPC endpoints are available on every node and can be accessed via HTTP and WebSocket. Most developer tools, dashboards, and libraries (like [Polkadot.js](/develop/toolkit/api-libraries/polkadot-js-api){target=\_blank}, [Subxt](/develop/toolkit/api-libraries/subxt){target=\_blank}, and others) utilize these endpoints internally.
+
+## How Do RPC Calls Work?
+
+RPC (Remote Procedure Call) is a protocol that allows you to invoke functions on a remote server (in this case, a blockchain node) as if they were local. Polkadot SDK nodes implement the [JSON-RPC 2.0](https://www.jsonrpc.org/specification){target=\_blank} standard, making it easy to interact with them using standard HTTP requests.
+
+```mermaid
+flowchart LR
+CLIENT([Client Application])-- JSON-RPC Request -->NODE([Node])
+NODE -- JSON Response --> CLIENT
+```
+
+RPC calls are stateless and can be used to:
+
+- Query chain state (e.g., block number, storage values)
+- Submit extrinsics (transactions)
+- Monitor node and network health
+- Retrieve metadata and runtime information
+
+## Making RPC Calls with Curl
+
+You can make RPC calls to a node using [`curl`](https://curl.se/){target=\_blank} or any HTTP client. The general format that the RPC calls stick to is the following:
+
+```bash
+curl -H "Content-Type: application/json" \
+  -d '{"id":1, "jsonrpc":"2.0", "method": "INSERT_METHOD_NAME", "params": [INSERT_PARAMS]}' \
+  NODE_ENDPOINT
+```
+
+- **`method`**: The RPC method you want to call (e.g., `system_health`).
+- **`params`**: Parameters for the method (if any).
+- **`NODE_ENDPOINT`**: The HTTP endpoint of your node (e.g., `http://localhost:9933` or a public endpoint).
+
+Here's a simple example to get the latest block number of the Polkadot relay chain; you can use the following node endpoint:
+
+```bash
+curl -H "Content-Type: application/json" \
+  -d '{"id":1, "jsonrpc":"2.0", "method": "chain_getBlock"}' \
+  https://rpc.polkadot.io
+```
+
+## Essential RPC Methods
+
+Below are some of the most useful and commonly used RPC methods for Polkadot SDK-based chains. Each method includes a description, parameters, and an example request.
+
+---
+
+### system_health
+
+Checks the health of your node.
+
+**Parameters:**
+
+None
+
+**Example:**
+
+```bash title="system_health"
+curl -H "Content-Type: application/json" \
+    -d '{"id":1, "jsonrpc":"2.0", "method": "system_health", "params":[]}' \
+    http://localhost:9933
+```
+
+---
+
+### chain_getBlock
+
+Returns the latest block or a specific block by hash.
+
+**Parameters:**
+
+- `blockHash` *(optional, string)* – The hash of the block to retrieve. If omitted, returns the latest block.
+
+**Example:**
+
+```bash title="chain_getBlock"
+curl -H "Content-Type: application/json" \
+    -d '{"id":1, "jsonrpc":"2.0", "method": "chain_getBlock", "params":[]}' \
+    http://localhost:9933
+```
+
+---
+
+### state_getStorage
+
+Queries on-chain storage by key (requires [SCALE-encoded](/polkadot-protocol/parachain-basics/data-encoding){target=_blank} storage key).
+
+**Parameters:**
+
+- `storageKey` *(string)* – The SCALE-encoded storage key to query.
+
+**Example:**
+
+```bash title="state_getStorage"
+curl -H "Content-Type: application/json" \
+    -d '{"id":1, "jsonrpc":"2.0", "method": "state_getStorage", "params":["0x..."]}' \
+    http://localhost:9933
+```
+
+---
+
+### author_submitExtrinsic
+
+Submits a signed extrinsic (transaction) to the node.
+
+**Parameters:**
+
+- `extrinsic` *(string)* – The SCALE-encoded, signed extrinsic (transaction).
+
+**Example:**
+
+```bash title="author_submitExtrinsic"
+curl -H "Content-Type: application/json" \
+    -d '{"id":1, "jsonrpc":"2.0", "method": "author_submitExtrinsic", "params":["0x..."]}' \
+    http://localhost:9933
+```
+
+---
+
+### state_getMetadata
+
+Fetches the runtime metadata (needed for decoding storage and extrinsics).
+
+**Parameters:**
+
+None
+
+**Example:**
+
+```bash title="state_getMetadata"
+curl -H "Content-Type: application/json" \
+    -d '{"id":1, "jsonrpc":"2.0", "method": "state_getMetadata", "params":[]}' \
+    http://localhost:9933
+```
+
+---
+
+## Check Available RPC Calls
+
+To check all the RPC methods exposed by your node, you can use the `rpc_methods` call to get a comprehensive list of available methods. This is particularly useful when working with different chain implementations or custom runtimes that may have additional RPC endpoints. You can do this via [`curl`](#using-curl) or the [Polkadot.Js Apps](#using-polkadotjs-apps).
+
+### Using curl
+
+To check the available RPC methods using `curl`, you can use the following command:
+
+```bash
+curl -H "Content-Type: application/json" \
+  -d '{"id":1, "jsonrpc":"2.0", "method": "rpc_methods", "params":[]}' \
+  https://rpc.polkadot.io
+```
+
+You can replace `https://rpc.polkadot.io` with the node endpoint you need to query.
+
+### Using Polkadot.js Apps
+
+1. Go to the [Polkadot.js Apps UI](https://polkadot.js.org/apps){target=\_blank} and navigate to the RPC calls section.
+
+    ![](/images/develop/toolkit/parachains/rpc-calls/rpc-calls-01.webp)
+
+2. Select **`rpc`** from the dropdown menu.
+
+    ![](/images/develop/toolkit/parachains/rpc-calls/rpc-calls-02.webp)
+
+3. Choose the **`methods`** method.
+
+    ![](/images/develop/toolkit/parachains/rpc-calls/rpc-calls-03.webp)
+
+4. Submit the call to get a list of all available RPC methods.
+
+    ![](/images/develop/toolkit/parachains/rpc-calls/rpc-calls-04.webp)
+
+This will return a JSON response containing all the RPC methods supported by your node.
+
+![](/images/develop/toolkit/parachains/rpc-calls/rpc-calls-05.webp)
+
+From this interface, you can also query the RPC methods directly, as you would do with curl.
+
+## Resources
+
+- [Polkadot JSON-RPC API Reference](https://polkadot.js.org/docs/substrate/rpc/){target=\_blank}
+- [Parity DevOps: Important Flags for Running an RPC Node](https://paritytech.github.io/devops-guide/guides/rpc_index.html?#important-flags-for-running-an-rpc-node){target=\_blank}
+- [Polkadot.js Apps RPC Explorer](https://polkadot.js.org/apps/#/rpc){target=\_blank}