Add CI workflows to docs source repos to check validity of docs

[Michiel-s]

No description provided.

[hanjoosten]

This is getting more urgent. @stefjoosten has proven that this is required. In the latest update he did, documents are missing. This should have been validated in a PR job.
See https://github.com/AmpersandTarski/AmpersandTarski.github.io/actions/runs/3538106570/jobs/5938644977 for a log that shows that such an error prevented the build of the documentation.

[hanjoosten]

The main goal for using docusaurus was to be able to keep the site ampersandtarski.github.io in sync with the features that are in production. For this reason, we planned to generate the one-pager site each time something is pulled into the main of each repo.

That sulution has a drawback: It doesn't allow for easy fixing of current documentation. The strict rules we have in place to ship to main are hindering.

For this reason I see an even better solution. Let's creat a specific branch in each repo that contains documentation that should be incorporated in the one-pager site. I propose a uniform name of such branch, documentation. The purpose of that branch will be that it contains the 'current' documentation. Fixes of documentation can be done in that branch. Whenever that branch is updated, that will trigger the build of docusaurus.
Whenever main is updated, we should (automatically?) merge to documentation, so the build of docusaurus is triggered.

This way of working will ensure that the documentation on the site is always up-to-date with the features in main. It also enables us to work on technical debt of documentation itself.