feat: adding a beginner-level HTML cheatsheet

[CBID2]

This PR adds an HTML cheatsheet for beginners.
Closes #3296

• At least two implementers are interested (and none opposed):
  • …
  • …
• Tests are written and can be reviewed and commented upon at:
  • …
• Implementation bugs are filed:
  • Chromium: …
  • Gecko: …
  • WebKit: …
  • Deno (only for timers, structured clone, base64 utils, channel messaging, module resolution, web workers, and web storage): …
  • Node.js (only for timers, structured clone, base64 utils, channel messaging, and module resolution): …
• MDN issue is filed: …

(See WHATWG Working Mode: Changes for more details.)

[annevk]

Hey @CBID2, thank you for writing this up. Unfortunately due to it not being automated the concerns in the issue about it going out-of-date still apply. I would encourage you to publish it elsewhere, be it a personal blog or MDN, as it might well be useful to others.