Don't enforce xvfb for generating the man pages

Summary:
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.

Test Plan:
Read and try the instructions.

./contrib/teamcity/build-configurations.py build-docs

On Archlinux:

ninja install-manpages

Should no longer fail.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7913