Better logging in ANTS

We should be able to make better use of logging in Ants. Current usage is pretty sparse and added to diagnose particular issues. If we can extend the current usage, it will make future debugging easier.

There's three main parts to this:

1. Checking that current usage is consistent with best practice (and updating if necessary). A reasonable reference for best practice is [​here](https://docs.python.org/3/howto/logging.html#logging-howto), but a better reference may be available.
2. Documenting current usage so end users can benefit.
3. Logging configuration entries should be case insensitive (i.e. `enabled=True` should be just as valid as `enabled=true`). This is inherited from a legacy issue so may have already been tackled.
4. After the building blocks are in place, extending logging to more parts of the code base.

These can be tackled in one ticket, or split into multiple tickets as appropriate.

Note that this is for the logging infrastructure in core ANTS. A separate issue exists for [contrib applications](https://code.metoffice.gov.uk/trac/ancil/ticket/2761) (MOSRS link - should be updated after contrib is moved to GitHub).

Start by reviewing the existing logging usage in core ANTS alongside the Python Logging HOWTO linked in the issue. Define the scope before changing it: consistent current usage, documentation for end users, case-insensitive logging configuration, and expanded coverage; completion should address the agreed subset and its validation.

Written by the indexing model from the issue text.