Add isopycnal mixing section to 25 km MOM6 parameter documentation

[minghangli-uni]

Issue description

We've reviewd and merged the MOM6 parameter choices for the 25km configuration in be4e095.

However, the isopycnal mixing section was deliberately excluded from the doc at this stage as we agreed that this section rquires further shared understanding and additional review before being documented in a definitive way.

Issue checklist

• I have labelled this issue with the affected configurations (e.g. MC_25km_jra_ryf, [1] more options)
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[1] some label options
MC_100km_jra_ryf
MC_100km_jra_iaf
MC_100km_jra_ryf+wombatlite
MC_25km_jra_ryf
MC_25km_jra_ryf+wombatlite
MCW_100km_jra_ryf
MCW_100km_jra_iaf
dev-MC_4km_jra_ryf+regionalpanan+isf
dev-MC_4km_jra_ryf+regionalpanan
all-configurations