Policy overrides require equivalent overrides in horizon

This bug tracker is for errors with the documentation, use the following as a template and remove or add fields as you see fit. Convert [ ] into [x] to check boxes:

- [x] This is a doc addition request.

When setting policyd-overrides for keystone, for example, we also need to add those policyd-overrides to openstack-dashboard. Does it make sense to mention that in the charm-guide? And maybe the reasons why? (Because there is no API call to list the policy and horizon is just a client that doesn't have special access other than API calls.) Ideally this should link to the charmhub docs for the openstack-dashboard charm where it mentions the directory structure for policy overrides in horizon but https://charmhub.io/openstack-dashboard has no docs. Instead I guess we can reference the README on the charm's repo: https://opendev.org/openstack/charm-openstack-dashboard#policy-overrides

It also occurs to me that this might be best added to the tutorial that is mentioned by this guide: https://docs.openstack.org/charm-guide/latest/admin/ops-show-extended-server-attributes.html

-----------------------------------
Release: 0.0.1.dev813 on 2023-07-01 00:22:03
SHA: 5483425c3d5b11ec5c39d68a8c42311b991e53ea
Source: https://opendev.org/openstack/charm-guide/src/doc/source/admin/policy-overrides.rst
URL: https://docs.openstack.org/charm-guide/latest/admin/policy-overrides.html