Docs infrastructure update to unpin from v. old Sphinx version

[sadielbartholomew]

Close #350.

Includes fixing some minor docs issues along the way (a few typos where cf. was used instead of cfdm., a broken text link, and some old class stubs that are no longer valid classes which broke the build).

The main fixes to get newer version of Sphinx to behave as we'd like as not mess up the API reference structuring, as per this PR are:

• to set autosummary_generate = False in the conf.py configuration, because when enabled that meant the docs/source/class/cfdm.*; class obect files with our custom lists structure we being overwritten by those generated from some templates;
• to provide a Jinja2 template for the base object in the location it expects, namely under a subdirectory _templates/autosummary/base.rst of the _templates location set in the conf.py (we had ours only in the parent _templates/ with no such directory) - note we need this still given autosummary further used in the custom class files we have, so we still rely on some templates to cover functions and method etc. stubs, even if we have our own top-level class reference RST files;
• change implicit labels such as :ref:`cfdm.Object <cfdm-Object>` to well-defined Python object references namely :py:class:`cfdm.Object` because the former were allowed in old Sphinx versions from being assumed/taken as implicit references, but newer versions are stricter so these aren't valid there;
• in our functions template, replace the auto-gen'd function Junja2 injection of {{ fullname }} with {{ objname }}, because under the new Sphinx versions and/or any the changes above, the linking on the list of functions in functions.rst had otherwise broken.

[davidhassell]

Hi Sadie,

Aside from my comment on tutorial.rst this looks great! Very many thanks for finally sorting this out :-)

[sadielbartholomew]

The second commit added, 4bb9cf1, fixes one more issue I spotted in final self-review of the docs generated with the new versions, namely that the functions pages weren't linking properly.

Merging without re-review so I can get on with preparing the new release (I trust that neither of my two commits since are controversial).