This brings us one step towards having docs for every tagged version.
For now (after the next tag push) there should only be one website for
the latest tagged version. Earlier versions can be built offline.
Progress to #199.

In any case, it is better than the current situation: html documentation
is being deployed for every single push on `main`, and the docs website
did not indicate a version number anywhere. With these GH workflow
adjustments, the docs should still be _built_ for each PR/push to main,
allowing to catch errors in the documentation setup as they arise,
but the html pages will not deployed to GitHub Pages if there is no tag.
The version (obtained from `git describe`) is also indicated clearly
on the website landing page and in the API docs sidebar footer.

Related things that I rolled in here:
- new make target to build docs (simply `make html` in the action spec)
- favicon and proper branding (multiple resolutions of logo)
- removed a duplicated sentence from the README
- `docs/template/README.md` to explain how to maintain the template
- fixed `rm -r` -> `rm -rf` in `make clean` recipe