RTL support via a single main style file

[chalin]

• Restart of and contribution to Reintroduce RTL support using BSv5 RTLCSS #1442. Recovers from changes to the following PRs:
  • Add RTL support using BSv5 #2002 - updates layouts/partials/head-css.html
  • [RTL] Fix top-nav position #2018 - drops RTL "adjustment", which is no longer necessary
• With this PR, each page is linked to a single style file, either the LTR version (main.css) or an RTL version (main.rtl.css), created from the LTR version piped through rtlcss
• Ran Prettier over Multi-linguage UG page, and cleaned up the front matter. No content changes yet.