Update docs building

Switch to "modern" way of building docs using sphinx-build directly,
remove now unsed parts from setup.cfg.

Upgrade to openstackdocstheme 1.20 and remove obsolete variables from
conf.py.

Convert external links to internal RST links so that Sphinx can verify
that they are correct.

Replace redirected links with new targets.

Use opendev.org instead of github.com where appropriate.

Change-Id: Iedcc008b170821aa74acefc02ec6a243a0dc307c
diff --git a/doc/source/conf.py b/doc/source/conf.py
index e9708fa..9059f8c 100644
--- a/doc/source/conf.py
+++ b/doc/source/conf.py
@@ -11,9 +11,6 @@
 # All configuration values have a default; values that are commented out
 # serve to show the default.
 
-import sys
-import os
-
 # If extensions (or modules to document with autodoc) are in another directory,
 # add these directories to sys.path here. If the directory is relative to the
 # documentation root, use os.path.abspath to make it absolute, like shown here.
@@ -26,13 +23,16 @@
 
 # Add any Sphinx extension module names here, as strings. They can be extensions
 # coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
-extensions = [ 'sphinx.ext.autodoc', 'zuul_sphinx', 'openstackdocstheme', 'sphinxcontrib.blockdiag', 'sphinxcontrib.nwdiag' ]
+extensions = [ 'sphinx.ext.autodoc',
+               'zuul_sphinx',
+               'openstackdocstheme',
+               'sphinxcontrib.blockdiag',
+               'sphinxcontrib.nwdiag' ]
 
 # openstackdocstheme options
 repository_name = 'openstack-dev/devstack'
 bug_project = 'devstack'
 bug_tag = ''
-html_last_updated_fmt = '%Y-%m-%d %H:%M'
 
 todo_include_todos = True
 
@@ -119,11 +119,6 @@
 # pixels large.
 #html_favicon = None
 
-# If not '', a 'Last updated on:' timestamp is inserted at every page bottom,
-# using the given strftime format.
-git_cmd = "git log --pretty=format:'%ad, commit %h' --date=local -n1"
-html_last_updated_fmt = os.popen(git_cmd).read()
-
 # If true, SmartyPants will be used to convert quotes and dashes to
 # typographically correct entities.
 #html_use_smartypants = True
@@ -167,17 +162,6 @@
 
 # -- Options for LaTeX output --------------------------------------------------
 
-latex_elements = {
-# The paper size ('letterpaper' or 'a4paper').
-#'papersize': 'letterpaper',
-
-# The font size ('10pt', '11pt' or '12pt').
-#'pointsize': '10pt',
-
-# Additional stuff for the LaTeX preamble.
-#'preamble': '',
-}
-
 # Grouping the document tree into LaTeX files. List of tuples
 # (source start file, target name, title, author, documentclass [howto/manual]).
 latex_documents = [