doc: Include static members in Doxygen

Summary:

This makes our Doxygen output more useful by generating it for static members.

This is a backport of Core PR16347

Test Plan: cmake .. -GNinja && ninja doc-doxygen

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8377