Documentation build performance

[jamesgardner1421]

Currently the documentation build takes more than 30 minutes on GitHub, and although faster locally, it can be quite annoying to update just because it takes so long to build. Other packages don't appear to have this problem so it is worth trying to figure out if this can be improved.

Possible options include:

• Splitting subpackage documentation (NQCModels.jl, CubeLDFAModel.jl). However, this might make it slightly harder to find stuff if it's all split up.
• Remove some extra docs dependencies. For example, we don't need to use both Plots and Makie to generate the plots, picking just one would help.
• Shorten some of the examples. Some of the ensemble simulations take a non-negligible amount of time to run and these add up. We could also consider caching the plots and not running the code at all though things might break without us realising.