Linear model documentation

[cjohnson-pi]

Are there any linked Issues or Pull Requests?

#516

Brief description

Existing documentation for the linear model is at:
https://code.metoffice.gov.uk/trac/lfric/browser/Docs/branches/dev/christinejohnson/r32587_tl_docs/Tangent_Linear

And for linear FFSL is attached to #414

This needs to be migrated to lfric_apps documentation

Further details of the issue.

No response