rft: eddy diffusivity docstrings

[haakon-e]

This pull request refactors and improves the implementation and documentation of the eddy diffusivity coefficient calculation and its application in the vertical diffusion boundary layer tendency routines. The main changes include moving coefficient helper functions to a more appropriate file, enhancing documentation, simplifying function signatures, and improving code clarity and maintainability.

Refactoring and code organization:

• Moved the definitions of eddy_diffusivity_coefficient_H and eddy_diffusivity_coefficient from precomputed_quantities.jl to eddy_diffusivity_coefficient.jl, consolidating all eddy diffusivity logic in one place. [1] [2]
• Inlined these helper functions into the main coefficient computation functions and updated their signatures for clarity and directness.

Documentation and interface improvements:

• Added detailed docstrings to ᶜcompute_eddy_diffusivity_coefficient for both DecayWithHeightDiffusion and VerticalDiffusion models, explaining the formulas, arguments, and usage.
• Simplified and clarified function signatures for vertical_diffusion_boundary_layer_tendency!, removing unnecessary arguments and improving readability.

Boundary layer tendency routine enhancements:

• Refactored the computation of divergence operators and enthalpy/trace diffusion to use clearer variable names, reduce redundancy, and improve maintainability. [1] [2]
• Unified the handling of divergence operators for energy and tracer diffusion, and streamlined the application of boundary conditions. [1] [2]