Core Docs Refactor and Redesign (#36067)

* Docs refactor as outlined in https://github.com/ansible/proposals/issues/79. Moves content into 'guides'; refactors TOC; fixes CSS; design tweaks to layout and CSS; fixes generated plugin, CLI and module docs to fix links accodingly; more.

* Adding extra blank line for shippable

docs/docsite/rst/user_guide/playbooks_python_version.rst

@@ -0,0 +1,69 @@
+.. _pb-py-compat:
+
+Python Version and Templating
+=============================
+
+Jinja2 templates leverage Python data types and standard functions.  This
+makes for a rich set of operations that can be performed on data.  However,
+this also means that certain specifics of the underlying Python becomes
+visible to template authors.  Since Ansible playbooks use Jinja2 for templates
+and variables, this means that playbook authors need to be aware of these
+specifics as well.
+
+Unless otherwise noted, these differences are only of interest when running
+Ansible in Python2 versus Python3.  Changes within Python2 and Python3 are
+generally small enough that they are not visible at the jinja2 level.
+
+.. _pb-py-compat-dict-views:
+
+Dictionary Views
+----------------
+
+In Python2, the :meth:`dict.keys`, :meth:`dict.values`, and :meth:`dict.items`
+methods returns a list.  Jinja2 returns that to Ansible via a string
+representation that Ansible can turn back into a list.  In Python3, those
+methods return a :ref:`dictionary view <python3:dict-views>` object.  The
+string representation that Jinja2 returns for dictionary views cannot be parsed back 
+into a list by Ansible.  It is, however, easy to make this portable by
+using the :func:`list <jinja2:list>` filter whenever using :meth:`dict.keys`,
+:meth:`dict.values`, or :meth:`dict.items`::
+
+    vars:
+      hosts:
+        testhost1: 127.0.0.2
+        testhost2: 127.0.0.3
+    tasks:
+      - debug:
+          msg: '{{ item }}'
+        # Only works with Python 2
+        #loop: "{{ hosts.keys() }}"
+        # Works with both Python 2 and Python 3
+        loop: "{{ hosts.keys() | list }}"
+
+.. _pb-py-compat-iteritems:
+
+dict.iteritems()
+----------------
+
+In Python2, dictionaries have :meth:`~dict.iterkeys`,
+:meth:`~dict.itervalues`, and :meth:`~dict.iteritems` methods.  These methods
+have been removed in Python3.  Playbooks and Jinja2 templates should use
+:meth:`dict.keys`, :meth:`dict.values`, and :meth:`dict.items` in order to be
+compatible with both Python2 and Python3::
+
+    vars:
+      hosts:
+        testhost1: 127.0.0.2
+        testhost2: 127.0.0.3
+    tasks:
+      - debug:
+          msg: '{{ item }}'
+        # Only works with Python 2
+        #loop: "{{ hosts.iteritems() }}"
+        # Works with both Python 2 and Python 3
+        loop: "{{ hosts.items() | list }}"
+
+.. seealso::
+    * The :ref:`pb-py-compat-dict-views` entry for information on
+      why the :func:`list filter <jinja2:list>` is necessary
+      here.