tox -e docs fails with "WARNING: RSVG converter command 'rsvg-convert' cannot be run. Check the rsvg_converter_bin setting"

NOTE: This is only applicable to stable/train

Since this change:

https://review.opendev.org/#/c/678779/

Local docs builds fail if you don't have librsvg2-bin installed for the sphinxcontrib-svg2pdfconverter dependency (I'm on Ubuntu 18.04). We should include that in bindep.txt.

Ref to similar bug in nova: https://bugs.launchpad.net/nova/+bug/1844583

Output of broken build:

Ref: https://zuul.opendev.org/t/openstack/build/04b588d7e79945cdb29fbfdd6d431f22

Warning, treated as error:
RSVG converter command 'rsvg-convert' cannot be run. Check the rsvg_converter_bin setting
ERROR: InvocationError for command /home/zuul/src/opendev.org/openstack/networking-ovn/.tox/docs/bin/sphinx-build -W -b html doc/source doc/build/html (exited with code 2)
docs finish: run-test after 12.42 seconds