add docs for staged configs phase 2

[vivki]

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[mjang]

returns metadata you can use to create a Staged Config through a POST.

This raises questions:

• Which POST? (which endpoint)
• How would you use the metadata to create a Staged Config? (Would it be enough to enter the metadata as a part fo the REST call, or do you have to do more?)

[mjang]

@vivki Thanks for the draft. I see (at least most) of what you're doing for import/export with the following "flag": "staged-configs-phase-2"

I presume that feature flag still blocks what's shown in the API docs. (You can see for yourself. Take your repo, and follow the steps in https://github.com/nginx/documentation/blob/main/CONTRIBUTING_DOCS.md. Run the make watch command, and you'll have a local build in localhost:1313.