[w3c/manifest] Editorial: merge priv/sec into one section (#962)

Closes #915

This change (choose at least one, delete ones that don't apply):

* Makes editorial changes (changes informative sections, or changes normative sections without changing behavior)

Commit message:
Relocates the privacy/security advice to a single priv/sec section.


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<a href="https://pr-preview.s3.amazonaws.com/w3c/manifest/pull/962.html" title="Last updated on Mar 4, 2021, 3:15 AM UTC (bd79a90)">Preview</a> | <a href="https://pr-preview.s3.amazonaws.com/w3c/manifest/962/603762e...bd79a90.html" title="Last updated on Mar 4, 2021, 3:15 AM UTC (bd79a90)">Diff</a>
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/962

-- Commit Summary --

  * Editorial: merge priv/sec into one section

-- File Changes --

    M index.html (130)

-- Patch Links --

https://github.com/w3c/manifest/pull/962.patch
https://github.com/w3c/manifest/pull/962.diff

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