Include all project requirements in doc/requirements.txt file
It was removed in [1] but it is needed to properly build documentation
on the readthedocs service.
Also, as requirements-check job is checking doc/requirements.txt file
and has troubles with parsing such included files, this patch proposes
new requirements file only for readthedocs builds. This new file simply
includes all other our requirements files.
[1] https://review.opendev.org/c/x/tobiko/+/769109
Change-Id: I428b23db94c65a6138e3d3039fb28bac24c8ea8b