CI test for link breakage

[bgilbert]

Add a non-required CI job that checks PRs for:

• Removal of existing pages
• Removal of existing anchors (~= headings) within a page

[lucab]

This relates to breakage like #160.

While the goal is indeed good (never breaking permalinks), I don't think we can realistically promise that "we won't change page names nor section title ever after initially merged" at this point.
This topic mixes with docs versioning too, because we wittingly traded in occasional breakage (including permalinks) for fresher / more refined content.

In the short term, I think that a more immediately valuable CI job is "all internal links are ok", and using more often Discourse as an intermediate anchor for those cases where we highly value link stability.

[bgilbert]

This relates to breakage like #160.

It was originally prompted by #148 (comment).

While the goal is indeed good (never breaking permalinks), I don't think we can realistically promise that "we won't change page names nor section title ever after initially merged" at this point.

That's why I proposed a non-required CI job. I agree that we'll have to refactor the docs as they expand, but I think it'd be useful to have a speed bump so folks don't accidentally break external links.