docs(index[toctree]): Add GitHub external link to sidebar

why: Surface the GitHub repo as a first-class nav item inline with
     the normal toctree entries.
what:
- Add GitHub <https://github.com/tmux-python/libtmux-mcp> to toctree
  in index.md

Author: tony

docs/index.md

@@ -119,4 +119,5 @@ glossary
 
 project/index
 history
+GitHub <https://github.com/tmux-python/libtmux-mcp>
 ```