Don't enforce xvfb for generating the man pages
D7677 made it mandatory to use xvfb for generating the man pages. This
causes a few issues:
- It enforces installing xvfb which comes with a lot of dependencies, even if bitcoin-qt is not built.
- xvfb-run is not designed to run parallel, and will fail on some systems due to race when running ninja install-manpages (even with the -a flag).
This diff removes the mandatory xvfb and adds a README to instruct the
user that it might be required and how to install it.
Read and try the instructions.
Should no longer fail.
Reviewers: #bitcoin_abc, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Differential Revision: https://reviews.bitcoinabc.org/D7913