[backport#15638] [docs] Document src subdirectories and different libraries

Summary:
https://github.com/bitcoin/bitcoin/pull/15638/commits/fd509bd1f71df628b933ea7a135a9a957a5e0136

Depends on D6000

This is a partial backport of Core PR15638

Test Plan: none

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6003