Sometimes we break links (full or partial) when we move pages about or rename headers, so now html-proofer will run as part of the build to make sure we've not broken any links.

- The Reliability Engineering section for CDIO no longer exists
- Remove call-to-action to update the manual as there is no corresponding section
- Re-add "Testing with RSpec" header for ruby to unbreak a link
- Update links after changes to source-code guidance structure

Sometimes pages disappear from the internet or move, and we want to keep our outbound links in good condition. We sleep between requests to GitHub to avoid being rate-limited, and ignore some specific repositories which will fail the link checker.

- The Oracle JDK licence page does not resolve outside of a browser
- The FeatureFlags.io article is gone, replace it with a page sourced from Martin Fowler's site
- Minifycode.com is gone, replace it with a link to Google's web dev perf advice
- Replace NIST publication about least privilege with similar page from NCSC

Also remove duplication between a single GitHub URL and the blanket GitHub regex