Combine AMP with PWA

[yogeshbeniwal]

Feature description

Continuing discussion from GoogleChromeLabs/pwa-wp#564, to consider below as recommended by AMP team at https://amp.dev/documentation/guides-and-tutorials/learn/combine-amp-pwa/

• AMP pages with PWA features
• AMP as entry point into PWA
• Use AMP as data source

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Acceptance criteria

Implementation brief

QA testing instructions

Demo

Changelog entry

[westonruter]

For AMP as entry point and data source in a SPA, see exploration in #1519. However, I don't see this PR being merged, as I don't see it as being in scope for this plugin.

Also, see issue for adding AMP content to REST API: #1014 (and PR #2827)