refactor: move GetTransaction(...) to node/transaction.cpp

Summary:
This removes a circular dependency caused by historical reasons

<sipa> GetTransaction predates node/transaction.cpp, and even the generic index framework itself

And also documentation follow-ups to D12208

This is a backport of core#22528

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12243