Make build and CI more contributor friendly

[chalin]

Tasks:

• Check formatting as a GH action #2445
• CI: add link-checking GH action, stop failing Netlify build on link-check issues #2504
• [CI] Disable cSpell inside code blocks for mermaid, and others? #3747
• Consider GH action on push, or something similar, for the refcache; although the following might be better:
  Create bot action to run formatter and link checker #2388
• Check for use of external links to site-local pages.
  Related: CI: warn about external links to local pages rather than abort with an error #2497

Related (CI enhanced checks):

• Check spelling via GH action? - CI: run spelling checker #948
• Add GH action to pull in latest release of the OTel specification #2387

[chalin]

All done, closing.