Add doc/requirements

We need to specify doc requirements in doc/requirements.txt
to avoid problems with the pip resolver [1] for the release team [2][3].
Removed specific doc requirements from test-requirements.txt.

The problem here is that this repos haven't doc/requirements.txt file
and by default in this case zuul will use the test-requirements.txt file
to pull requirements [4].

This requirements file contains extra requirements like flake8 that
collided with those allowed in our job environment and so the new pip
resolver fails to install these requirements and the job exits in error.

This project meet the conditions leading to the bug however it doesn't
produce any doc or releasenotes, but uniformization can't hurt and help
us in the future.

/!\/!\/!\
Notice that I voluntarily added the doc directory even if no docs
are generated here because zuul will try to pull this requirements from
there first and the contained requirements are needed for reno but AFAIK
the releasenotes dir is ignored by zuul. c.f [4] for further details.
/!\/!\/!\

[1] http://lists.openstack.org/pipermail/release-job-failures/2021-January/001500.html
[2] http://lists.openstack.org/pipermail/openstack-discuss/2021-January/019611.html
[3] http://lists.openstack.org/pipermail/openstack-discuss/2021-January/019612.html
[4] https://opendev.org/zuul/zuul-jobs/src/branch/master/roles/ensure-sphinx/tasks/main.yaml#L36

Change-Id: I18fdf133b6c3ea9b60e784c0e1dfb7ce1217eda5

doc/requirements.txt

@@ -0,0 +1,3 @@
+sphinx!=1.6.6,!=1.6.7,>=1.6.2  # BSD
+openstackdocstheme>=1.11.0  # Apache-2.0
+reno>=1.8.0 # Apache-2.0

test-requirements.txt

@@ -10,10 +10,6 @@ pyflakes>=2.1.1 # MIT
 
 coverage>=4.0,!=4.4 # Apache-2.0
 python-subunit>=0.0.18 # Apache-2.0/BSD
-sphinx!=1.6.6,!=1.6.7,>=1.6.2  # BSD
 oslotest>=1.10.0 # Apache-2.0
 stestr>=1.0.0 # Apache-2.0
 testtools>=1.4.0 # MIT
-openstackdocstheme>=1.11.0  # Apache-2.0
-# releasenotes
-reno>=1.8.0 # Apache-2.0