Add instructions for building Doxygen docs

Summary: Doxygen works without any changes. Added a note about building it locally. The doxygen docs are already in .gitignore, so they won't be added to the repo.

Test Plan: doxygen doc/Doxyfile

Reviewers: schancel, deadalnix, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Subscribers: deepthawtz, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1302