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.