[w3c/manifest] Chore: Use figures for the mask examples (#852)

This change (choose one):

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

Commit message:

Converts the "Mask example" images to use `<figure>`/`<figcaption>` based on comments here: https://github.com/w3c/manifest/pull/833#discussion_r356526848

You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/852

-- Commit Summary --

  * Chore: Use figures for the mask examples

-- File Changes --

    M index.html (131)

-- Patch Links --

https://github.com/w3c/manifest/pull/852.patch
https://github.com/w3c/manifest/pull/852.diff

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