Updating 2.x support materials redesign release

On 13 October 2021 (https://www.w3.org/2020/10/13-ag-minutes.html) Hidde 
de Vries presented to AG a project about redesigning the Understanding 
and Techniques for WCAG 2.x, as part of a larger redesign and 
consolidation of WAI resources. We are preparing to deploy the updated 
materials soon. There are no changes to content for these materials, 
they are still published from the same sources. Page organization and 
design have changed, summarized by Hidde:

  * clearly mark these documents as part of W3C (logo), part of WAI
    (colors, layout, fonts), part of WCAG (WCAG is the first word on the
    page)
  * show clearly which set of documents the user is looking at,
    including which version of WCAG they belong to (2.1, 2.2 etc)
  * have clear pointers to other supporting documents and the general
    “All WCAG Guidance”
  * use a lay-out that is also used by other supporting docs (soon: ACT
    Rules, COGA Design Guide)
  * visually distinguish between name of the document (eg “Using
    aria-label for link purpose”), how it fits in the larger set
    (“Sufficient to meet 2.4.4”), the actual document content, meta
    information (eg links to other sources, summary of key terms,
    definition of the document type)
  * invite people to contribute (“Help improve this page”)
  * make code examples easier to use (with syntax highlighting)

The updated will be deployed by merging PR 2012 
https://github.com/w3c/wcag/pull/2012 though this is just implementation 
code.

The most important links to review are the previews in the PR:

Techniques: https://draft-wcag-redesign.netlify.app/techniques/
Understanding: https://draft-wcag-redesign.netlify.app/understanding/

In those locations you will see the entire suites in their new format. 
Some links may be broken because of the staging location, but otherwise 
this should be ready to review. Please send any comments by reply to 
this message, or as comments in the PR. We intend to deploy within the 
next few weeks, timed with other dependent projects.

Michael