Add Go Custom Code Page (#59)

Author: devalog

fern/products/sdks/overview/go/custom-code.mdx

@@ -1,8 +1,102 @@
 ---
 title: Adding custom code
-description: Augment your TypeScript SDK with custom utilities
+description: Augment your Go SDK with custom utilities
 ---
 
-Learn how to extend your Fern Go SDK with custom code and utilities.
+<Markdown src="/products/sdks/snippets/custom-code-intro.mdx"/>
 
-<Warning>This page is a WIP, please refer to our previous [documentation](https://buildwithfern.com/learn/sdks/capabilities/custom-code).</Warning>
+## Adding custom logic
+
+<Markdown src="/products/sdks/snippets/custom-logic-intro.mdx"/>
+
+<Steps>
+
+    ### Create a new file and add your custom logic
+    
+      ```go title="helper.go"
+      func MyHelper() {
+        fmt.Println("Hello World!")
+      }
+      ```
+
+    ### Add your file to `.fernignore`
+
+    <Tip>A `.fernignore` file is automatically created in your SDK repository when you use GitHub publishing.</Tip>
+
+
+    ```yaml {3} title=".fernignore"
+    # Specify files that shouldn't be modified by Fern
+
+    helper.go
+    ```
+
+    ### Consume the helper
+
+    <Markdown src="/products/sdks/snippets/consume-method.mdx"/>
+
+    ```go
+      import "github.com/package/example"
+
+      example.MyHelper();
+    ```
+</Steps>
+
+    ## Adding custom SDK methods
+
+    <Markdown src="/products/sdks/snippets/custom-sdk-methods-intro.mdx"/>
+
+    <Steps>
+      ### Update `generators.yml` configuration
+      Name your Fern-generated client something like `BaseClient` to reflect that this client will be extended.
+
+      ```yml {4} title="generators.yml"
+      - name: fernapi/fern-java-sdk
+          version: "..."
+          config:
+            client-class-name: BaseClient
+      ```
+
+      ### Import and extend the generated client
+
+      First, import the Fern generated base client and extend it. Then, add whatever methods you want.
+
+      ```go title="client/my_client.go"
+      type MyClient struct {
+        *Client // Embed the Fern generated client.
+      }
+
+      func NewMyClient(opts ...option.RequestOption) *MyClient {
+        return &MyClient{
+          Client: NewClient(opts...),
+        }
+      }
+
+      func (m *MyClient) MyHelper() {
+        fmt.Println("Hello World!")
+      }
+      ```
+
+      ### Update `.fernignore`
+
+      Add the `client/my_client.go.` to `.fernignore`.
+
+        ```diff title=".fernignore"
+        + client/my_client.go
+        ```
+
+      ### Consume the method
+
+      Instead of constructing the generated client, your users will want to construct the extended client.
+
+      ```go title="main.go"
+      import exampleclient "github.com/package/example/client"
+
+      client := exampleclient.NewMyClient():
+      ```
+
+      <Markdown src="/products/sdks/snippets/consume-method.mdx"/>
+
+      ```go
+      client.MyHelper()
+      ```
+    </Steps>