Add intersphinx Sphinx extension

Add the intersphinx Sphinx extension to enable seamless
linking from the charm-guide.

This is already enabled for the charm-guide - for seamless
linking from this guide (deploy-guide).

Change-Id: I36442bfc8ff9267786cce87b32dfff2348c30f57

deploy-guide/source/conf.py

@@ -36,7 +36,14 @@ import os
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
 # ones.
 # TODO(ajaeger): enable PDF building, for example add 'rst2pdf.pdfbuilder'
-extensions = ['openstackdocstheme']
+extensions = [
+    'openstackdocstheme',
+    'sphinx.ext.intersphinx'
+]
+
+intersphinx_mapping = {
+    'cg': ('https://docs.openstack.org/charm-guide/latest', None)
+}
 
 # Add any paths that contain templates here, relative to this directory.
 # templates_path = ['_templates']