Fix 404s in docs and in repo READMEs

Created by: sqs

A user @P3GLEG reported that our docs and various repository READMEs have many broken links. We need to have an automated way to identify and prevent these errors.

• Identify and fix all current 404s in doc pages
• Identify and fix all current 404s in repository READMEs
• Identify and fix all current 404s otherwise in our repositories (e.g., in comments, in blog posts, etc.)
• Monitor all of those sources continuously for regressions (links becoming 404s)