doc: Add installation guides for Zed and OpenCode

[ChihweiLHBird]

Summary

Please add official installation guides for Zed and OpenCode (similar to the existing guides for Cursor, Windsurf, Cline, Roo Code, etc.) in docs/installation-guides/.

These two modern AI-native editors/tools have growing MCP support and active users trying to connect to the official GitHub MCP Server, but there is currently no dedicated guide.

Why this matters

• OpenCode: Multiple users are already attempting to use the GitHub MCP Server with OpenCode and running into connection/auth issues (see Unable to connect to GitHub MCP Server from within OpenCode #1396 and [BUG] windows - Incompatible auth server: doe not support dynamic client registration - OpenCode #2344). A clear guide would reduce friction and help surface the correct configuration (Docker vs stdio, PAT setup, remote server options).
• Zed: Zed has strong native context server / MCP support. While a community extension exists (LoamStudios/zed-mcp-server-github), many users still want to use the official github-mcp-server binary or Docker image directly. An official guide would improve consistency and discoverability.
• The repo already maintains high-quality guides for similar popular hosts. Adding Zed and OpenCode would keep the documentation comprehensive as more editors adopt the MCP standard.