[w3c/manifest] Add protocol handlers to the explainer and spec (#863)

Closes #846

This change (choose one):

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

Implementation commitment (delete if not making normative changes):

* [ ] Safari (link to issue)
* [x] Chrome ([link to issue](https://bugs.chromium.org/p/chromium/issues/detail?id=1019239))
* [ ] Firefox (link to issue)
* [x] Edge ([public signal](https://github.com/MicrosoftEdge/MSEdgeExplainers/tree/master/URLProtocolHandler))

Commit message:
Add protocol handlers to the explainer and spec.

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

  https://github.com/w3c/manifest/pull/863

-- Commit Summary --

  * Add protocol handlers to the explainer and spec

-- File Changes --

    M explainer.md (20)
    M index.html (195)

-- Patch Links --

https://github.com/w3c/manifest/pull/863.patch
https://github.com/w3c/manifest/pull/863.diff

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