[doc] Add examples of u-c-r, u-c-c and user.rc

As of today we do support very powerful set of overrides for roles,
collections and environment variables that are not widely used or known
by deployers.

This patch aims to add some example files to sample openstack_deploy
folder to raise awareness about existence of these files.

Change-Id: I3940f5277c7d5a26c2686382758507c19b0c815d

ansible-role-requirements.yml.example

@@ -1,6 +0,0 @@
-# Use this file to fill in your third party roles that you'd like to have added to the list of available roles.
-# Example:
-#  - github_api: https://api.github.com/repos/os-cloud/opc_role-galera_client
-#    name: galera_client
-#    src: https://github.com/os-cloud/opc_role-galera_client
-#    version: master

etc/openstack_deploy/user-collection-requirements.yml.example

@@ -0,0 +1,9 @@
+# Use this file to fill in your third party collections or roles that you'd like to have added during bootstrap-ansible.sh.
+# Collections with the same name will have prescedence and override ones provided in ansible-collection-requirements.yml
+# Content of this file will be fed to ansible-galaxy binary, so it should be compatible with requirements.yml format.
+# Example:
+#   collections:
+#   - name: openstack.cloud
+#     source: https://github.com/os-cloud/ansible-collections-openstack
+#     type: git
+#     version: main

etc/openstack_deploy/user-role-requirements.yml.example

@@ -0,0 +1,7 @@
+# Use this file to fill in your third party roles that you'd like to have added during bootstrap-ansible.sh.
+# Roles with the same name will have prescedence and override ones provided in ansible-role-requirements.yml
+# Example:
+#  - name: galera_client
+#    src: https://github.com/os-cloud/opc_role-galera_client
+#    scm: git
+#    version: master

etc/openstack_deploy/user.rc.example

@@ -0,0 +1,4 @@
+# Use this file to define any needed environment variables that will be sourced
+# by openstack-ansible binary after loading main RC fail.
+# Example:
+#    export ANSIBLE_INVENTORY="${ANSIBLE_INVENTORY},/opt/extra/inventory.py"