Improve WRF documentation for surface obs interpolation and boundary condition perturbation

[braczka]

Use case

Give WRF-DART users a better introduction to the operation of the forward operator process for surface observations. Inferring the inner-workings of the forward operator from the code itself is very challenging.

Also, add more documentation to the implementation of ensemble generation (perturbation) process. The use of the WRFDA CV3 option for the WRF domain is already covered a bit, but the perturbation of the boundary conditions (application of tendencies) is not covered and mostly hidden in the code.

Is your feature request related to a problem?

Users can usually get the WRF-DART tutorial to work, but are challenged when customizing for their own application, because some fundamental concepts are not covered.

Describe your preferred solution

Devote a documentation section/paragraph specifically to an overview of forward operation functionality, and also how ensemble spread is generated.

Describe any alternatives you have considered

Do nothing -- continue to respond to users through email support.