Add errata to w3c/wcag repo and cross-reference from informative docs

[kfranqueiro]

This adds the following:

• Imports the 2.1 and 2.2 errata pages into this repo
  • Reorganizes existing 2.1 errata in reverse-chronological order
  • Adds date stamps for all existing errata I could trace back to a commit
  • Makes use of variables and Liquid expressions to minimize potential for copy-paste errors between sections
  • Auto-generates table of contents within CustomLiquid.ts to avoid desync when adding versions
  • Documents patterns/formats for authoring errata in errata/README.md (with mention in top-level README)
• Adds logic to parse information from errata pages and include relevant errata within Guideline/SC boxes and term definitions within informative docs pages
  • Errata are only included when building for a specific version (i.e. not the editor's draft, in which case corrections are expected to already be inlined)
  • Only errata against the latest published version are included in informative docs
• Updates publish-w3c script to also copy relevant errata file for version being published
• Adds links to errata pages in top-level index used for dev server and PR builds

Examples of how errata appear:

For a success criterion (as an additional subheading):

For a key term definition (as an additional paragraph + list):

[w3cbot]

kfranqueiro marked as non substantive for IPR from ash-nazg.

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