Link to detail on LFRic naming conventions

### What problem does your feature request solve?

To help new users we should provide a link/indication of what LFRic naming conventions are at point of use within rose edit, on the lists of variables to process.

### Describe the solution you'd like

Our current definition is based on the [_stash_to_lfric.py](https://github.com/MetOffice/CSET/blob/main/src/CSET/operators/_stash_to_lfric.py) file, which isn't that human readable and only contains limited information. It would be good to link to a more authoritative source, however I'm not sure if one exists.

### Describe alternatives you've considered

> Not sure of anywhere myself other than the spreadsheet and the STASH2LFRic file. Might be worth checking with someone from RAD. If we can't find anything I am happy for the conversations to be marked as resolved provided we open an issue so that it gets put in place for solving it when we know more.
> \- @daflack in https://github.com/MetOffice/CSET/pull/1055#discussion_r1924023632

> Also unsure, suspect its local information stored by Ian Boutle/Paul Earnshaw et al., as LFRic is still mostly being tested
> \- @jwarner8 in https://github.com/MetOffice/CSET/pull/1055#discussion_r1925538018

Start by reviewing src/CSET/operators/_stash_to_lfric.py and the rose edit variable-processing list described in the issue. Check the spreadsheet, STASH2LFric material, and consult RAD to identify an authoritative naming-conventions source. Done means the point of use links to or indicates that source.

Written by the indexing model from the issue text.