docs: Clean up redundant NCMEC docs

[cassidyjames]

There was a lot of duplicated content between the (already very long) user guide and the dedicated NCMEC docs page. Instead, let's summarize it for the user guide, then link to the dedicated docs page where relevant.

It also looks like the NCMEC reporting docs previously got put in the middle of the user roles section; instead, let's move it below so it makes a little more sense. :)

Rendered: https://github.com/roostorg/coop/blob/cassidyjames/docs-ncmec/docs/USER_GUIDE.md

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Follow-up to #169, as mentioned in #169 (comment)

[julietshen]

This is probably fine, but I do want to ask anyone in the community if they'd prefer a little redundancy in order to have more surface areas where they can figure out how to impelement NCMEC reporting