CI: changes to README.md are not pushed to gh-pages

[etiennebacher]

Looks like the CI workflow pushes almost all changes to gh-pages, but doesn't update the README.md there. This commit was made by the CI after I renamed plot2 to tinyplot on main, but it didn't include the changes I made in the README.

Sidenote: I tried to set clean: true when deploying to gh-pages in the altdoc CI. This is a bad idea as it will erase all previous files in this branch. In this case, since the README file was not pushed automatically, gh-pages had no README and therefore the website had no homepage (I had to manually commit the README to gh-pages). Moral: better to have an outdated homepage than no homepage => leave clean: false

Mentioned in #258