Update examples & docs (esp. API reference) pre 3.5.0 release

[sadielbartholomew]

Close #119 and #120 by updating examples with datasets which correspond to more than one Field when read-in, both in the test suite and in the docs - including adding a 'tip' admonition to explain to users that there are different ways to filter FieldLists and the consistent approach the examples take for the choice used.

More generally, update the docs also in preparedness for a v3.5.0 release in January, via:

• updating remaining reference images for test suite for Missing space in cbar labels between variable name & units #115 fix now that ggap.nc examples fail w/ downloaded data due to indexing #120 has been fixed such that the tests generate the correct plot with the gap between name and units included (full test suite now passes again);
• remove internal API from API reference now it is obsolete;
• add sub-headings to categorise the functions in the API reference instead of having them all in one big list;
• use autosummary to list functions in API reference - they were listed as standard TOC tree pages from Andy's docs, hence the weird doubled item listing.

and also as left-over bits from previous PRs:

• add the validate module which I somehow missed committing in .
• update the Changelog with recent updates.

A bit of a bumper PR but in the interests of time left before Christmas I've put this all in one rather than separate PRs - individual commit messages should help to classify changes relative to the above list.