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/vcs-python/vcspull> to toctree in index.md

Author: tony

docs/index.md

@@ -95,4 +95,5 @@ internals/index
 project/index
 history
 migration
+GitHub <https://github.com/vcs-python/vcspull>
 ```