Add documentation for mesoscale forcing capabilities

[ewquon]

This PR documents the mesoscale-to-microscale coupling strategies that were introduced by #652. These strategies provide mesoscale forcing through source terms based on:

1. Tendencies (or mesoscale budget components)
2. Profile assimilation — generalized to include "direct" and "indirect" forcing schemes, applied to the full simulation domain or a portion thereof.

[marchdf]

Thank you for doing this! One suggestion I might have is to refer to this documentation in the "theory manual" part of the documentation:https://exawind.github.io/amr-wind/theory/theory.html#source-terms We have a section there on GravityForcing in the "source terms" section. Just a short section/sentence with a cross reference of sort would probably be plenty.

[ewquon]

Good suggestion! I'll add some text shortly.

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[mbkuhn]

Planning to merge this PR when the other one gets merged because I'm assuming this corresponds to the changes in the other PR. If I'm mistaken on that, I could merge this one first.