Add MOM6 instructions to Relative_Vorticity.ipynb

[adele-morrison]

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 move to Mains-Advanced

Also remove cf-xarray/pint.

[navidcy]

#606 moves this recipe into Mains

[chrisb13]

It's further down the road but I chatted to @navidcy and @aekiss about this and I think in this instance it would be worth having a separate notebook for mom6 in this case (and still keep the old one). They are useful examples of how b/c grids involve different calculations.