add MkDocs as documentation framework

[DaveSkender]

Convert our Ruby/Jekyll documentation site in the /docs folder cleanly into an equivalent MkDocs site. The high level tasks are to:

• Add MkDocs
• Remove Jekyll site entirely
• Migrate all basic site configurations from the old Jekyll site to the new MkDocs site
  • HTML meta, SEO, and web vitals configs
  • Handle production vs preview site
• Update build/deploy GitHub Action workflows for doc site for
  • Accessibility (a11y) testing
  • HTML URL checker
  • Website deployment
  • Add/remove GitHub secrets and variables as needed
• Migrate all content from the old site to the new site
• Remove all obsolete prior site code