docs: add copilot howto

Author: daiyam

docs/ext-github-copilot.md

@@ -0,0 +1,22 @@
+<!-- order: 16 -->
+
+# Extension: GitHub Copilot
+
+Unlike Visual Studio Code, in VSCodium, Copilot features are disabled and not configured.
+
+## Update your settings
+
+In your settings, sets:
+```
+"chat.disableAIFeatures": false,
+```
+
+## Configure product.json
+
+You need to create a custom `product.json` at the following location (replace `VSCodium` by `VSCodium - Insiders` if you use that):
+- Windows: `%APPDATA%\VSCodium` or `%USERPROFILE%\AppData\Roaming\VSCodium`
+- macOS: `~/Library/Application Support/VSCodium`
+- Linux: `$XDG_CONFIG_HOME/VSCodium` or `~/.config/VSCodium`
+
+Then you will need to follow the guide [Running with Code OSS](https://github.com/microsoft/vscode-copilot-chat/blob/main/CONTRIBUTING.md#running-with-code-oss) with the `product.json` file created previously.
+You will need to add the properties: `trustedExtensionAuthAccess` and `defaultChatAgent`.

docs/index.md

@@ -25,6 +25,7 @@
   - [Proprietary Extensions](https://github.com/VSCodium/vscodium/blob/master/docs/extensions.md#proprietary-extensions)
   - [Using the "VSIX Manager" Extension](https://github.com/VSCodium/vscodium/blob/master/docs/extensions.md#vsix-manager)
   - [Extensions compatibility](https://github.com/VSCodium/vscodium/blob/master/docs/extensions-compatibility.md)
+  - [Ext: GitHub Copilot](https://github.com/VSCodium/vscodium/blob/master/docs/ext-github-copilot.md)
 - [Migration](https://github.com/VSCodium/vscodium/blob/master/docs/migration.md)
   - [Manual Migration from Visual Studio Code to VSCodium](https://github.com/VSCodium/vscodium/blob/master/docs/migration.md#manual-migration)
   - [Semi-Automatic Migration with "Sync Settings" Extension](https://github.com/VSCodium/vscodium/blob/master/docs/migration.md#semi-automatic-migration)