new 'NRAI postmortems' doc

[ying-nr]

This pr basically based off from a branch (reviewed by @paperclypse)
#7471

Update from @zuluecho9: no release date for this yet. We said that @paperclypse would review it week of May 9th.

What problems does this PR solve?
PR includes a doc page for NRAI new postmortems feature (not released yet).
At current stage, we are trying to get the doc in reviewable state

Add any context that will help us review your changes such as testing notes,
It includes 7 images with postmortem- prefix
The doc draft (with images) has been initially reviewed by @barbnewrelic 

[github-actions]

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[paperclypse]

We're still waiting on a release date for this one.

[akristen]

Looks good! Thank you!