Add packages required for pdf-docs run to bindep.txt
Add packages required for pdf-docs run to bindep.txt

This adds packages needed to run 'tox -e pdf-docs' locally to our
bindep.txt for users, for example:

  bindep -l newline pdf-docs | xargs <package manager> install -y

The CI jobs don't use the 'pdf-docs' label for handling bindep for pdf
docs builds [1] and instead install a separate list of prereqs [2], so
this change is only relevant for local test runs.

[1]https://opendev.org/openstack/openstack-zuul-jobs/src/commit/64341d2e322b9e1c7f197bb0aafd63038d4e925c/zuul.d/jobs.yaml#L435
[2]https://opendev.org/openstack/openstack-zuul-jobs/src/commit/64341d2e322b9e1c7f197bb0aafd63038d4e925c/roles/prepare-build-pdf-docs/tasks/main.yaml#L7-L20

Change-Id: Id5553242b5bb013c42bb01fd389d43d71c8b78ea

openstack/nomad