[sadielbartholomew]

Adds a table of contents listing all examples in numbered order (as well as an index page overall TOC), since previously they could only be accessed via themed pages. For a more self-contained structuring, I've made an individual file with a sub-title containing the code snippet and displaying the image (in better RST formatting) which can be included in those themed pages, or in any useful place, via the ..include:: directive, as applied throughout this PR. I have also added titles and contents to each example for some further context.

This PR includes (re)assignment of (sub-)numbering for some docs examples, since there were some with duplicate numbers e.g. there were two different code blocks called 'Example 21' with two different plots, plus some had no numbering and just a descriptive title. Now every example has a unique number, though to avoid disparate numbering of similar examples some are assigned 'a, b, c,' etc.

Not pushing a re-build of the HTML docs yet, until all bug fixes are in to fix the remaining incorrect plots.

Follow-on work

After this PR, at least I want to also, pre-release:

• Structure general TOC listed pages, esp. so the API reference pages are organised under a sub-heading;
• Remove the manually-configured left sidebar in favour of the top-level one provided by the theme;
• Bug fixes for examples producing plot images which are incorrect.

[sadielbartholomew]

Merging now since this PR will become too bloated if I add more docs fix-up/improv. and I need to get the bug fixes merged...