docs: about gh-pages

[faretek1]

make clear what the gh-pages does - and its volatile nature

[faretek1]

The gh-pages branch is generated by a github action and is force-pushed every single time there is a push made to this repository. This means that the gh-pages branch should NOT be edited by hand, because the data will be lost the next time someone pushes to the main branch.

If you want to add a non-markdown file to the site, use docs/hooks.py