docs: Add doc page for GitHub Copilot model access

aider/website/docs/llms/github.md

@@ -0,0 +1,83 @@
+---
+parent: Connecting to LLMs
+nav_order: 510
+---
+
+# GitHub Copilot models
+
+Aider can talk to the GitHub Copilot LLMs because Copilot exposes an **OpenAI-compatible** REST
+API at `https://api.githubcopilot.com`.
+
+The only trick is getting an OAuth access token that has permission to call the Copilot
+endpoint.  
+The easiest, **official** way is to sign in to Copilot from any JetBrains IDE
+(Goland, PyCharm, etc).  
+After you sign in, a file appears at:
+
+```
+~/.config/github-copilot/apps.json
+```
+
+Inside you will find an `oauth_token` value – copy that string, **it is your API key**.
+
+---
+
+## Configure the environment
+
+```bash
+# macOS/Linux
+export OPENAI_API_BASE=https://api.githubcopilot.com
+export OPENAI_API_KEY=<oauth_token from apps.json>
+
+# Windows (PowerShell)
+setx OPENAI_API_BASE https://api.githubcopilot.com
+setx OPENAI_API_KEY  <oauth_token>
+# …restart the shell so the variables are picked up
+```
+
+---
+
+## Pick a model
+
+Copilot hosts many models (OpenAI, Anthropic, Google, etc).  
+You can discover the list that your subscription allows with:
+
+```bash
+curl -s https://api.githubcopilot.com/models \
+  -H "Authorization: Bearer $OPENAI_API_KEY" \
+  -H "Content-Type: application/json" \
+  -H "Copilot-Integration-Id: vscode-chat" | jq -r '.data[].id'
+```
+
+The returned IDs are used exactly like OpenAI models, but **prefixed with `openai/`** when you
+pass them to aider:
+
+```bash
+aider --model openai/gpt-4o
+# or
+aider --model openai/claude-3.7-sonnet-thought
+```
+
+You can also store this in `~/.aider.conf.yml`:
+
+```yaml
+openai-api-base: https://api.githubcopilot.com
+openai-api-key:  "<oauth_token>"
+model:           openai/gpt-4o
+weak-model:      openai/gpt-4o-mini
+show-model-warnings: false
+```
+
+---
+
+## Notes & FAQ
+
+* Copilot billing is handled entirely by GitHub.  Calls made through aider count against your
+  Copilot subscription, not a separate OpenAI bill.
+* Tokens created by **Neovim copilot.lua** or older `hosts.json` files sometimes lack the
+  required scopes. If you get `access to this endpoint is forbidden`, regenerate the token via a
+  JetBrains IDE or VS Code Copilot extension.
+* The Copilot terms of service allow third-party “agents” that access the LLM endpoint.  Aider
+  merely follows the documented API and **does not scrape the web UI**.
+
+Happy hacking!