Document how the website is updated for releases

[fingolfin]

Right now, just looking at this repository it is not clear how things are updated for a releases. We should document this in the repository, even if it just means pointing to docs somewhere else.

Some specific ideas:

• the README.md should talk about it
• relevant is of course the dev/releases/update_website.py script in the gap repository, so mention it, and also its README
• all generated files should have at their top a comment which indicates they are generated, should not be modified by hand, and ideally also how they were generated. E.g. "DO NOT EDIT THIS FILE MANUALLY. It was generated by the update_website.py script from the gap repository"
• pointers at the relevant CI jobs might also be helpful

[fingolfin]

If all goes well then the sync workflow will detect a new release and open a PR here. Then it can be merged. This leaves updating docs.gap-system.org and files.gap-system.org -- I just left a comment in #250 which discusses how this could then be triggered by merging that PR