Docs: clean build to update & purge superfluous JS etc.

[sadielbartholomew]

Follow-on to #33 where I committed the cf-plot documentation source with generated content pre-built by Andy Heaps, where I have made some tweaks so I (or indeed, anyone) can generate it locally with my conda environment and then re-built the documentation, in doing so picking out files which don't need to be included in the repo (mostly Javascript files for the theming under docs/bulid/_static and now apparently unused images under docs/_images.

We shouldn't generally re-build the documentation until release-time, but this is important to ensure the source and the built content are in sync, ready for updates. I have tested that this all looks as it should from my local fork of the repo using GitHub Pages.

Also included in the this PR (see also individual commit descriptions), since they were necessary for the build and/or as updates e.g. to account the new docs home under NCAS-CMS:

• fix to the setup.py to refer to the new README and not break on looking for the old README.txt;
• updating the introductory content to align with the new README for consistency;
• convert some hard-coded links to internal links so it won't point back to Andy's docs set instead of this one.

Direct follow-on work identified

• There are numerous Sphinx warnings and errors raised, so I will work through those separately next.
• Add link to Andy's built documentation for 3.3.0 to the 'Older documentation' page, as a link to the final generated copy under his user space.

[sadielbartholomew]

Admin PR so no review required. Merging since everything has been tested as displaying as it should on my user space and the file line change stats look fine.