Add Extensions API blog post for documentId and friends.

[dtapuska]

• Instant Navigation adds additional fields documentId, parentDocumentId, documentLifecycle, frameType to help improve the experience for back/forward cache and prerendering.

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[dotproto]

@jpmedley, FYI I'm tagging you in on this PR a little early. I figure It might be worth seeing how it reads & flows.

[jpmedley]

@dotproto,

Thank you! Much appreciated.

[jpmedley]

@dtapuska,

The review Simeon asked me to do is very meta. When the text is locked I'll do a copy edit (review for typos, spelling, punctuation, etc.)

[jpmedley]

@dtapuska, @dotproto

the structure of this looks pretty good. I made a few comments, but won't do a thorough edit until you you tell me the text is locked.

[dtapuska]

@dotproto ping, PTAL

[jpmedley]

@dotproto Is there anything I can do to help you move this?

[jpmedley]

@dotproto If you have no more comments, please approve this. I'll do my review then.