Add guide for GitHub MCP server with token exchange via Vault #651
Description
Summary
Create a guide demonstrating how to connect a GitHub MCP server through the gateway, using Vault for token exchange/management.
Motivation
From refinement discussion on #388:
"a guide focused on a github MCP server example with token exchange via vault would be a good addition here."
We already have docs/guides/external-mcp-server.md and config/samples/remote-github/ covering external MCP server setup, but no guide covering the Vault-based token exchange flow, which is a more realistic production pattern.
Scope
- Guide covering GitHub MCP server setup with Vault token exchange
- Show how to store a GitHub PAT in Vault and reference it for token exchange
- Link to external Vault docs for installation/configuration (not a Vault tutorial)
- Build on the patterns in the authorization guide, which covers JWT claims, AuthPolicy, and tool-level access control
- Reference existing external MCP server guide and remote-github samples