Simplify and add MOM6 instructions to Hovmoller_Temperature_Depth.ipynb

[adele-morrison]

There is no need for this recipe to show both temperature and salinity. Remove salinity code and plot.

Add documentation at top of recipe with MOM6 variable names and any grid/diagnostic differences that users need to be aware of if they want to convert the recipe on their own to use MOM6 output.

Also remove cf-xarray/pint.