[w3c/manifest] editorial: enable MDN annotations (#772)

Adds MDN annotation for various things, like so:

![Screenshot 2019-07-18 17 48 04](https://user-images.githubusercontent.com/870154/61439094-3d41a400-a984-11e9-9e8d-ee7a764d95b3.png)

Note: this only covers the BIP interface at the moment, as well as the CSS rule. 

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).

Implementation commitment (delete if not making normative changes):

* [ ] Safari (link to issue)
* [ ] Chrome (link to issue)
* [ ] Firefox (link to issue)
* [ ] Edge (public signal)

Commit message:

(Fill in. If making normative changes, describe exactly what the behavioral
difference will be.)
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/772

-- Commit Summary --

  * editorial: enable MDN annotations

-- File Changes --

    M index.html (1)

-- Patch Links --

https://github.com/w3c/manifest/pull/772.patch
https://github.com/w3c/manifest/pull/772.diff

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