Link to dev setup guide from README and CONTRIBUTING (#8827)

# Description
Today the README does not link to how to get started making `cargo
build` succeed. I've made some minor edits making the repo a bit more
"welcoming" to devs who wanna get started building the repo.

## Changes
* Update `README.md` on how to get started with development (making
`cargo build` succeed) by linking to [this
guide](https://docs.polkadot.com/develop/parachains/install-polkadot-sdk/)
* Update `CONTRIBUTING.md` with the same guide
* Generalize `DOCUMENTATION_GUIDELINES.md` to not only be about
Substrate.

---------

Co-authored-by: Bastian Köcher <[email protected]>

Author: Sajjon

README.md

@@ -24,6 +24,10 @@ If you want to get an example node running quickly you can execute the following
 curl --proto '=https' --tlsv1.2 -sSf https://raw.githubusercontent.com/paritytech/polkadot-sdk/master/scripts/getting-started.sh | bash
 ```
 
+## 👩🏽‍💻 Building
+
+In order to build this project you need to install some dependencies, follow the instructions in [this guide](https://docs.polkadot.com/develop/parachains/install-polkadot-sdk).
+
 ## 📚 Documentation
 
 * [Polkadot Documentation Portal](https://docs.polkadot.com)

docs/contributor/CONTRIBUTING.md

@@ -10,6 +10,10 @@ via pull-requests and need to be approved by the maintainers.
 > **Note:** Contributors who are part of the organization do not need to fork the repository. They can create a branch
 > directly in the repository to send a pull request.
 
+## How?
+
+In order to build this project you need to install some dependencies, follow the instructions in [this guide](https://docs.polkadot.com/develop/parachains/install-polkadot-sdk).
+
 ## Rules
 
 There are a few basic ground-rules for contributors (including the maintainer(s) of the project):

docs/contributor/DOCUMENTATION_GUIDELINES.md

@@ -1,12 +1,12 @@
-# Substrate Documentation Guidelines
+# Documentation Guidelines
 
-This document is focused on documenting parts of Substrate that relate to its external API. The list of such crates can
+This document is focused on documenting parts of the Polkadot SDK that relate to its external API. The list of such crates can
 be found in [CODEOWNERS](/.github/CODEOWNERS). Search for the crates auto-assigned to the `docs-audit` team.
 
 These crates are used by external developers and need thorough documentation. They are the most concerned with FRAME
 development.
 
-- [Substrate Documentation Guidelines](#substrate-documentation-guidelines)
+- [Documentation Guidelines](#documentation-guidelines)
   - [General/Non-Pallet Crates](#generalnon-pallet-crates)
     - [What to Document?](#what-to-document)
       - [Rust Docs vs. Code Comments](#rust-docs-vs-code-comments)