Make linkcheckbroken required to pass on PRs #2107

Currently make linkcheckbroken allows PRs with broken links to be merged. This is not OK. It makes Plone documentation look amateurish, like someone's hobby project. Yes, we're volunteers, but come on, let's take some pride. There's no good reason to introduce broken links and references, especially when we have automated tools to help us.

Of course, when external entities change their URLs or block linkcheck, we need to fix them separately.

This also needs to be enforced in the submodules, so we can catch broken links before they get merged in those repositories:

• documentation Require linkcheckbroken to pass on all pull requests #2113
• volto Require linkcheck to pass volto#8436
• plone.api Require linkcheck to pass. plone.api#618
• plone.restapi Require linkcheck to pass plone.restapi#2053
• training Require linkcheck to pass in CI training#1020
• aurora Require linkcheck to pass in CI aurora#245