doc: mention code variables relating to 'calc_dragio'

[phil-blain]

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At the end of the "Ocean" section, we mention that 'dragio' can be computed from 'iceruf_ocn' and 'thickness_ocn_layer1', but do not mention these code variables, nor the 'calc_dragio' setting used to activate that computation. Mention the code variables next to their mathematical symbol, for more clarity.

Closes #463

[phil-blain]

I added the readthedocs config file, it seems it is now required (I copied the one from CICE).

[apcraig]

@phil-blain, thanks for adding the yml file. This was working last week, not sure why it's suddenly not. I probably had a plan to fix this in Icepack when I updated the CICE yml and that task got lost on the list. Odd that it was working fine in Icepack until this week. Anyway, thanks.

I will leave it to others to comment on the documentation changes.