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

doc/readthedocs_requirements.txt

@@ -0,0 +1,6 @@
+# As openstack job build-reno-releasenotes and Read The Docs page support only
+# one single requirements file for a project, we need to have one such file
+# which will include all project requirements
+-r ../requirements.txt
+-r ../extra-requirements.txt
+-r ./requirements.txt

doc/requirements.txt

@@ -2,8 +2,5 @@
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
 
-# As openstack job build-reno-releasenotes and Read The Docs page support only
-# one single requirements file for a project, we need to have one such file
-# which will include all project requirements
 reno>=3.2.0                   # Apache-2.0
 sphinx>=3.4.2                 # BSD