Create longer "how-to" guide for scientists in model development

As discussed after reading comments on PR #284, these users will likely want to run CMEW multiple times, with different configurations, and may want to name their runs (among other considerations).

This documentation will be more in-depth than the Quick Start guide, though it should still link to external documentation when appropriate in order to minimise maintenance.

User guide: https://metoffice.github.io/CMEW/user_guide/index.html

Implementation ideas (in the order of priority):
1. How To guide
2. Tutorial

Explore [diataxis.fr](diataxis.fr) for this.

Start by reviewing the existing CMEW User guide and Quick Start guide, then consult the linked Diátaxis guidance. Create a longer How To guide for scientists covering repeated runs with different configurations and naming runs, linking to external documentation where appropriate; a tutorial is the lower-priority alternative.

Written by the indexing model from the issue text.