Add Rossby localization documentation

[shlyaeva]

Description

All in the title

Checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have run the unit tests before creating the PR

[travissluka]

ugh, who put that Rossby radius localization in there without any associated documentation!?!? Whoever it was, that person no longer deserves to oversee soca!!

[guillaumevernieres]

Doc looks good, but there's an issue with the equation rendering:

When that happens, I ask my friend to make the markdown equation to render on gihub, it usually does the trick.

[travissluka]

I'm rescinding my previous approval:
Now that sequential EnKF is merged in we should add a short compatibility note? A user who picks Rossby loc for EAKF will hit a runtime ABORT from ObsLocRossby::computeLocalization(Point3, Point3).

Suggested addition:

Solver compatibility

Rossby localization currently works only with LETKF / GETKF. It is not yet supported with sequential EnKF solvers (e.g. EAKF), which require obs–obs localization. rossby_radius is only defined on the model grid, not at arbitrary obs pairs. Invoking it from a sequential solver aborts at runtime.