docs(index[toctree]): Add MCP and GitHub external links to sidebar

why: Surface the MCP server and GitHub repo as first-class nav items
     directly below Glossary, inline with the normal toctree entries.
what:
- Add MCP <https://libtmux-mcp.git-pull.com> to toctree in index.md
- Add GitHub <https://github.com/tmux-python/libtmux> to toctree

Author: tony

docs/index.md

@@ -105,4 +105,6 @@ project/index
 history
 migration
 glossary
+MCP <https://libtmux-mcp.git-pull.com>
+GitHub <https://github.com/tmux-python/libtmux>
 ```