[DOC] Various updates to cmake/ninja

Summary:
This diff migrates the remaining documentation (after D5394 and D5395)
to cmake and ninja.

It also completes the unit tests documentation with the seeder part.

Test Plan: Read the docs.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5397