Document style file syntax

### What problem does your feature request solve?

The style files added in #570 are largely undocumented, which makes it hard to write new ones.

### Describe the solution you'd like

A page should be added to the [Technical Reference](https://metoffice.github.io/CSET/reference/index.html) section of the documentation, similar to the existing [Recipe Format](https://metoffice.github.io/CSET/reference/recipe-format.html) page. This should have a reference to the format of the style file (JSON), the meaning of the various keys, and some examples showing how it can be structured.

### Describe alternatives you've considered

We don't document it, and no one knows how to write CSET style files except @cjohnson-pi.