doc: Start using openstackdoctheme's extlink extension

This ensures we have version-specific references to other projects [1].
Note that this doesn't mean the URLs are actually valid - we need to do
more work (linkcheck?) here, but it's an improvement nonetheless.

[1] https://docs.openstack.org/openstackdocstheme/latest/#external-link-helper

Change-Id: I118e4d211617c5df66ff04dc04e308a1d2fc67ad

doc/source/user/quickstart.rst

@@ -367,8 +367,8 @@ There are a few options that are required to deploy Kolla-Ansible:
   User has to specify images that are going to be used for our deployment.
   In this guide `DockerHub <https://hub.docker.com/u/kolla/>`__ provided
   pre-built images are going to be used. To learn more about building
-  mechanism, please refer `image building documentation
-  <https://docs.openstack.org/kolla/latest/admin/image-building.html>`_.
+  mechanism, please refer :kolla-doc:`Building Container Images
+  <admin/image-building.html>`.
 
   Kolla provides choice of several Linux distributions in containers:
 
@@ -444,8 +444,9 @@ There are a few options that are required to deploy Kolla-Ansible:
 
      neutron_external_interface: "eth1"
 
-  To learn more about network configuration, refer `Network overview
-  <https://docs.openstack.org/kolla-ansible/latest/admin/production-architecture-guide.html#network-configuration>`_.
+  To learn more about network configuration, refer
+  :kolla-ansible-doc:`Network overview
+  <admin/production-architecture-guide.html#network-configuration>`.
 
   Next we need to provide floating IP for management traffic. This IP will be
   managed by keepalived to provide high availability, and should be set to be
@@ -470,8 +471,8 @@ There are a few options that are required to deploy Kolla-Ansible:
   `a list of available services
   <https://github.com/openstack/kolla-ansible/blob/master/README.rst#openstack-services>`_.
   For more information about service configuration, Please refer to the
-  `Services Reference Guide
-  <https://docs.openstack.org/kolla-ansible/latest/reference/index.html>`_.
+  :kolla-ansible-doc:`Services Reference Guide
+  <reference/index.html>`.
 
 * Virtual environment
 
@@ -535,7 +536,7 @@ accordingly.
 
 When this playbook finishes, OpenStack should be up, running and functional!
 If error occurs during execution, refer to
-`troubleshooting guide <https://docs.openstack.org/kolla-ansible/latest/user/troubleshooting.html>`_.
+:kolla-ansible-doc:`troubleshooting guide <user/troubleshooting.html>`.
 
 Using OpenStack
 ~~~~~~~~~~~~~~~