Better documentation for configuring ANTS

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# 📚 Documentation Request

## Addition
The ANTS configuration options are in the [API docs](https://metoffice.github.io/ANTS/lib/ants.html#module-ants.config). It would be good if:

1. These were more prominent and discoverable. Some of these configuration settings are too important to be buried this far in the docs.
2. The docs for each config option look good. But pulling them out into the main docs rather than a code block would enable e.g. linking to a configuration option in response to a support request.

## Clarification

In general, the configuration options documentation could use a bit of a refresh. This is separate from any code restructuring to e.g. consolidate the use of configuration files, environment variables, and command line arguments in an application interface refresh.

Start with the configuration section in the ANTS API docs at ants.html#module-ants.config and inspect how the current options are presented. Identify the main documentation location where configuration guidance belongs, then make the options more discoverable and individually linkable without restructuring configuration code. Done means the refreshed documentation clearly exposes the options outside the current code block and preserves their existing descriptions.

Written by the indexing model from the issue text.