Review and refine ancil_general_regrid documentation

# 📚 Documentation Request

We should review/refine the `ancil_general_regrid` documentation so it is standalone rather than in need of supplemental information/pages.

We should also take care to review summary pages in the main docs so it is clear that fill operations are also being carried out - not just a regrid!

Start with the ancil_general_regrid documentation and the summary pages in the main docs. Review whether the material stands alone without supplemental pages and ensure the summaries explain that fill operations occur as well as regridding; the work is done when those documentation gaps are addressed.

Written by the indexing model from the issue text.