Multiple docs improvements: fix warnings + increase cross-referencing.

[pp-mo]

This is currently written "on top of" the "dim_slicer" PR.
So, WIP until that is done.

Lots of changes here, but all quite small.

• Mostly from a quick review of the utilities' APIs + their special docs-page added in Dim slicer #120.
• Gave each utils-page entry a reference to follow
• got the utils to list in source-order on their API page
• fixed the persistent build warnings

[pp-mo]

TODO?

This could be the time to re-phrase the "project purpose" descriptions

Current README says

Generic NetCDF data in Python.
Provides fast data exchange between analysis packages, and full control of storage formatting.
Especially : Ncdata exchanges data between Xarray and Iris as efficiently as possible"

Docs main-page says

NetCDF data interoperability.
Represents generic netcdf data as Python objects, equivalent to actual data files, but independent and freely modifiable.
Also supports translation to and from common analysis packages, so that data can be freely exchanged between packages without data copying.
... In particular Ncdata exchanges data between Xarray and Iris as efficiently as possible.

We might do better now to start by emphasizing the ability to transform + modify data,
and make the "data interchange" a secondary goal.
- but in the meantime, have not given up on #95. This just needs a little time spent on it.