Combine and simplify Compare_SSH_model_obs.ipynb and Compare_SST_SSS_TemperatureSalinity_to_WOA13.ipynb

[adele-morrison]

Simplify these recipes to only use one model and combine to compare both with WOA and SSH observations.

Also 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.

[chrisb13]

Related: https://github.com/COSIMA/cosima-recipes/issues/549#issuecomment-3404155223