Add "metric", "recipe" and "assessment area" to the glossary

Add definitions of "metric", "recipe" and "assessment area" to the [glossary in the documentation](https://github.com/MetOffice/CMEW/blob/main/doc/source/glossary.rst):

- "metric": a number that quantifies model performance
- "recipe": a grouping of metrics. If metrics are needed for a single plot, they must be grouped together in a single recipe. A recipe may contain one or more keithograms.
- "assessment area": a thematic aspect of model performance; a collection of metrics and plots relating to that thematic aspect, owned by a single domain expert. An "assessment area" contains at least one recipe and multiple metrics.

Context to justify the definitions above, not for the glossary... For the visualisation:
- Each recipe is displayed on its own page. Therefore, if an assessment area contains more than one recipe, each recipe will be displayed on a separate page.
- Each recipe must have a primary plot defined, which is used as a thumbnail. In most cases, this will be a keithogram.

**Details**

- Estimate: <1 day