Two small additions to make it easier for readers to find their way back
to the GitHub side of the project (stars, Discussions, PRs):
- overrides/main.html: site-wide announce banner with links to the repo
and Discussions. Uses Material's announce block, dismissable by the
reader once they've seen it
- mkdocs.yml: enable custom_dir for the override and add
announce.dismiss to theme features
- docs/index.md: replace the brief Contributing paragraph with a
Get Involved card grid (Star, Discussions, PRs, Issues). Sits on the
home page where most first-time visitors land
No content changes outside of these blocks.
