Fix pdf-docs build failing for TeX over capacity

With the new release of sphinx 3.1.0, nova pdf docs build
started failing with "! Dimension too large." error.
That started failing since 10th June when the requirement added
the new constraint for sphinx.

Seems like somewhere TeX memory is exhausted during the pdf
building (I think we are hitting this open sphinx bug[1]).
While reproducing it locally I found that our giant policy sample
file inclusion in pdf doc causing this error.

- https://zuul.opendev.org/t/openstack/build/9c3e835ad5ee4842a07d77fdbaa6c97d/log/sphinx-build-pdf.log#7661

We did skip the sample policy file for pdf in
doc/source/configuration/index.rst but did not do that
in admin configuration file and it start giving the error now.

With this fix, sample policy file in admin config also is included
in html but not in pdf.

Closes-Bug: #1883200

Change-Id: Iae143997138a5169a1e0fc76a74f9a0f09c03626

doc/source/admin/configuration/samples/index.rst

@@ -8,5 +8,14 @@ Files in this section can be found in ``/etc/nova``.
    :maxdepth: 2
 
    api-paste.ini
-   policy.yaml
    rootwrap.conf
+
+.. # NOTE(gmann): Keep policy sample file for HTML only.
+   # Sample file are too large and cause TeX memeor issue.
+   # ref bug# https://bugs.launchpad.net/nova/+bug/1883200
+.. only:: html
+
+   .. toctree::
+      :maxdepth: 2
+
+      policy.yaml