[docs] Use extlink for deploy guide references

Instead of using hardcoded links in our deploy guides, we replace them
with extlink sphinx extension. This is also a requirement from CI jobs.

Change-Id: I85de52647574d10bc52e16f75b2b804a425ebc45

deploy-guide/source/index.rst

@@ -9,7 +9,7 @@ intended for deployers.
 .. note::
 
    If you want to do a quick proof of concept of OpenStack, read the
-   `All-In-One quickstart Guide <https://docs.openstack.org/openstack-ansible/latest/user/aio/quickstart.html>`_
+   :dev_docs:`All-In-One quickstart Guide <user/aio/quickstart.html>`
    instead of this document. This document is a walkthrough of a deploy
    using OpenStack-Ansible, with all its configurability.
 

deploy-guide/source/next-steps.rst

@@ -8,11 +8,11 @@ is working, here is what you can do next:
 Operate OpenStack-Ansible
 =========================
 
-Review our `Operations guide <https://docs.openstack.org/openstack-ansible/latest/admin/index.html>`_
+Review our :dev_docs:`Operations guide <admin/index.html>`
 to learn about verifying your environment in more detail, and creating your first networks, images, and instances.
 
 Contribute to OpenStack-Ansible
 ===============================
 
-Review our `Contributors guide <https://docs.openstack.org/openstack-ansible/latest/contributor/index.html>`_
+Review our :dev_docs:`Contributors guide <contributor/index.html>`
 to learn about contributing to OpenStack-Ansible.

deploy-guide/source/overview.rst

@@ -6,8 +6,7 @@ Overview
 
    For essential background reading to help understand the service and storage
    architecture, please read
-   `OpenStack-Ansible Architecture in its reference guide
-   <https://docs.openstack.org/openstack-ansible/latest/reference/architecture/index.html>`_
+   :dev_docs:`OpenStack-Ansible Architecture in its reference guide <reference/architecture/index.html>`
    If you'd like to understand when OpenStack-Ansible would be a good fit for your
    organisation, please read the appendix
    ":ref:`app-aboutosa`".