[backport#16912] doc: Move doxygen intro to file for USE_MDFILE_AS_MANPAGE

Summary:
With USE_MDFILE_AS_MAINPAGE, this moves the introductory Doxygen comment to its own file. This makes bitcoind.cpp cleaner.

It also removes the \mainpage header text, which was smaller than the section titles, and improves the Navigation section.

This is a backport of Core PR16912

Test Plan: ninja doc-doxygen

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8061