doc: Doxyfile.in: trigger error on warning

This enforce warnings as errors when make doc is run.

Add a step before building start, checking that documentation hasn't been broken. This is made before build, deferring every other targets, so that a typo in documentation isn't built on the different targets, which should reduce contention in the end.

The MR won't pass until !3892 (merged), !3891 (merged) and !3890 (merged) are merged, but I open this now to discuss whether we should enable this unconditionnally or only in the CI.