Better document usage of masks in ANTS

# 📚 Documentation Request

## Addition
We need to have a section in our published docs discussing the usage of masks. In particular, we need to disambiguate numpy masks and land sea masks.

For numpy/dask masked arrays, this should include when they are used in ANTS, and how they should be filled (i.e. either via fill missing points, or filling with IMDI/RMDI as appropriate).

For land sea masks, this should include that land/sea masks use 1 and 0 to denote land and sea points (and when 1 means land, and when it means sea), and be explicit that this is not the same as numpy masked values.

Code conventions should also be extended with 'best practice' for working with numpy masks in ANTS, and in setting up ANTS tests.

Locate the published documentation and code-conventions sections, then review existing guidance on numpy/dask masked arrays, land-sea masks, and ANTS tests. Add the requested distinctions, filling guidance, value conventions, and best practices; done means the published docs clearly distinguish both mask types and explain the relevant test setup.

Written by the indexing model from the issue text.