[w3c/manifest] Adding in some accessibility-related language (#898)

Closes #860

This change (choose one):

* [ ] Breaks existing normative behavior (please add label "breaking")
* [ ] Adds new normative requirements
* [ ] Adds new normative recommendations or optional items
* [x] Makes only editorial changes (only changes informative sections, or
  changes normative sections without changing behavior)
* [ ] Is a "chore" (metadata, formatting, fixing warnings, etc).

Commit message:

Taking a crack at adding accessibility info. The only outstanding item is adding a recommendation for screenshot `label`s once ImageResource is integrated.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/898

-- Commit Summary --

  * adding accessible name & description calculation
  * adding prefers color scheme
  * adding prefers color scheme for theme_color

-- File Changes --

    M index.html (52)

-- Patch Links --

https://github.com/w3c/manifest/pull/898.patch
https://github.com/w3c/manifest/pull/898.diff

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