Improve documentation of masking and halo_removal

**the masking syntax need clarifying (and is possibly wrong)**

The example given in the documentation doesn't seem to have a "stride"

https://github.com/MetOffice/CDDS/blob/574dd77aa358d6d8eadbb6e617547d830353308f/docs/tutorials/mip_convert.md?plain=1#L270-L277
as described by the comment in

https://github.com/MetOffice/CDDS/blob/574dd77aa358d6d8eadbb6e617547d830353308f/mip_convert/mip_convert/new_variable.py#L414

Or in the model.json

https://github.com/MetOffice/CDDS/blob/574dd77aa358d6d8eadbb6e617547d830353308f/cdds/cdds/common/plugins/cmip6/data/model/UKESM1-0-LL.json#L214-L216

**confirm if masking can apply to any stream**

It isn't immediately clear to me whether you can apply masking to any stream or if there is logic in `_apply_mask` that restricts this to `cice`.

**order of masking and halo_removal operations is not explicitly documented**

Probably a rare occurrence but it should probably be made clear in the documentation what order masking and halo removal are applied.

https://github.com/MetOffice/CDDS/blob/574dd77aa358d6d8eadbb6e617547d830353308f/mip_convert/mip_convert/new_variable.py#L373-L374

Start with docs/tutorials/mip_convert.md and compare its masking example with mip_convert/mip_convert/new_variable.py, especially _apply_mask and the masking and halo-removal operations. Check cdds/cdds/common/plugins/cmip6/data/model/UKESM1-0-LL.json for the model syntax. Done means the documentation accurately explains the stride, supported streams, and operation order.

Written by the indexing model from the issue text.