GHA: the doc job isn't triggered when doc/Makefile is modified

[kit-ty-kate]

See #6807

[rjbou]

Discussed in dev meeting: As #6805, we'll need to see how to do those, if we should add modified files checks for a lot of jobs to be able to change the path trigger (the generator program helps with that), or split the main yaml file into several, but for that we need to be able to include a workflow file into another.