Port Wilson-Ballard microphysics scheme documentation into lfric_apps

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

_No response_

### Brief description

Documentation for the Wilson-Ballard microphysics scheme currently only exists in the UMDPs and is specific to the UM. This needs to be migrated to lfric_apps, to allow various in-progress changes to the microphysics scheme in lfric_apps to be properly documented.

At this stage we are just porting the existing UM microphysics scheme documentation without making significant changes. Further work will need to be done to tidy it up and remove UM-specific content that isn't applicable in LFRic, but this can be addressed under subsequent PRs.

The UMDPs are written in latex, whereas the new lfric_apps science documentation is to be written in Sphinx / ReStructureText, so a wholesale reformatting of the source is required. To port the UMDP document, I have followed the steps documented here: [Porting-UMDPs-to-LFRic](https://github.com/MichaelWhitall/lfric_apps/wiki/Porting%E2%80%90UMDPs%E2%80%90to%E2%80%90LFRic).

lfric_apps branch: [vn3.1_port_wbmphys_doc](https://github.com/MichaelWhitall/lfric_apps/tree/vn3.1_port_wbmphys_doc)

Copy of the built doc: [UMDP26.html](https://wwwspice/~michael.whitall/vn3.1_port_wbmphys_doc_html/science_guide/microphysics_schemes/UMDP26.html)

Original latex-based PDF of the equivalent UMDP for comparison: [umdp_026.pdf](https://upgraded-adventure-jznrmqp.pages.github.io/papers/umdp_026.pdf)

### Further details of the issue.

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