Add cron-like docs-build workflow

[staticdev]

• Here is the implementation of my proposed solution with addition of some of suggestions from @cjolowicz in our discussion on the issue.
• It has a CRON job trigger every week and also runs when there are changes on /docs and .rst files.
• It is on a separate workflow just for docs and so I could remove docs-build and upload from tests workflow.
• I've been testing it on a project running everyday and seems to work well.
• I had to ignore some local generated files because of a bug on Sphinx linkcheck Linkcheck marks local links broken when they reference files produced later, during in HTML building sphinx-doc/sphinx#9383
• Extra: I changed the "Read the Docs" badge to show the status of the new GA workflow in an attempt to have a fail faster approach and visibility of this weekly CRONjob.

Update: user guide needs to be addressed on a new PR on cookiercutter repo.

Closes cjolowicz/cookiecutter-hypermodern-python#901

[staticdev]

@cjolowicz could review current state here? From my side it is complete.

[cjolowicz]

Hey @staticdev thanks for working on this, it's on my radar, just haven't gotten around to reviewing it yet.