Simply the use of multi-panel figures in ported UMDPs

### Are there any linked Issues or Pull Requests?

_No response_

### Brief description

The reST code to generate figures in ported UMDP documentation is very convoluted. This is in order to cope with the possibility of multiple panels within a single figure.

The current approach is documented here: [Porting-UMDPs-to-LFRic](https://github.com/MichaelWhitall/lfric_apps/wiki/Porting%E2%80%90UMDPs%E2%80%90to%E2%80%90LFRic).

Alternative methods are discussed here: https://github.com/MetOffice/lfric_apps/pull/450#discussion_r3154186697

### Further details of the issue.

_No response_