Trigger docs rebuild on main repo change #2

Currently the docs website is only rebuilt when something changes in this repo.

However, it should be rebuilt if the source files change. Those live primarily in the simplesamlphp/simplesamlphp repo (but, to make things complicated, also sometimes in module repos).

Some possible solutions:

• Trigger this repo's workflow when a commit (including docs changes?) is done in the ssp main repo. And/or do the same from the module repos.
• Just rebuild this repo periodically (daily/weekly).