Add Spinx RTD theme

The RTD theme [1] is being used by x/tobiko [2] project
to generate its documentation page [3]

[1] https://github.com/readthedocs/sphinx_rtd_theme
[2] https://opendev.org/x/tobiko
[3] https://tobiko.readthedocs.io

- Is the library actively maintained?
  It is. On Sep 21th we had the last release (1.0.0)
  There are many recent commits on master branch:
    https://github.com/readthedocs/sphinx_rtd_theme/commits/master

- Is the library good code?
  This is a widely used sphinx plugin used to generate
  many documentation pages. It has been working without
  any problem to generate Tobiko doc page since years:

- Is the library python 3 compatible?
  It is:
    https://sphinx-rtd-theme.readthedocs.io/en/stable/development.html#supported-dependencies

- Is the library license compatible?
  It is being distributed under The MIT License:
    https://github.com/readthedocs/sphinx_rtd_theme/blob/master/LICENSE

- Is the library already packaged in the distros we target (Ubuntu latest / Fedora latest)?
  It is:
    https://packages.ubuntu.com/hirsute/all/sphinx-rtd-theme-common
    https://src.fedoraproject.org/rpms/python-sphinx_rtd_theme

- Is the function of this library already covered by other libraries in global-requirements.txt?
  The default OpenStack Sphinx theme is not allowed to be used by non official OpenStack projects
  Therefore Tobiko switch to use the RTD one that is the one provided by the same page where
  Tobiko's documentation is hosted:
    https://tobiko.readthedocs.io/

- Is the library required for OpenStack project or related dev or infrastructure setup? Which?
  Yes. It is being used to generate Tobiko project documentation page:
    https://opendev.org/x/tobiko/src/branch/master/doc/source

- If the library release is managed by the Openstack release process does it use the cycle-with-intermediary release type?
  This plugin is not managed by the Openstack release process.

- Do I need to update anything else?
  It doesn't. Release 1.0.0 only has below requirements (both already added):
    c9b1bde560/setup.py (L120-L123)
    - sphinx>=1.6
    - docutils<0.18

Change-Id: Ic75b781ce842e93ed4fe4730abbe79a108af3dc5

global-requirements.txt

@@ -464,6 +464,7 @@ sphinx-feature-classification  # Apache-2.0
 sphinxcontrib.datatemplates  # BSD License
 sphinxcontrib-programoutput  # BSD license
 sphinxcontrib-svg2pdfconverter  # BSD License
+sphinx-rtd-theme  # MIT
 stestr!=2.3.0,!=3.0.0  # Apache-2.0
 sushy!=1.9.0  # Apache-2.0
 tabulate  # MIT

upper-constraints.txt

@@ -606,6 +606,7 @@ XStatic-D3===3.5.17.0
 actdiag===2.0.0
 sysv-ipc===1.1.0
 sphinxcontrib-applehelp===1.0.2
+sphinx-rtd-theme===1.0.0
 scikit-learn===0.24.2;python_version=='3.6'
 scikit-learn===1.0.1;python_version=='3.8'
 setuptools===59.1.1