PI-3483 Ensure documentation of ancillary variables handling

[pp-mo]

Documentation to answer user questions like

"what happens to a cubes ancillary variables when I ..."

A broad variety of solutions is possible

• do nothing, allow users to determine by experiment
• write general account of strategies (e.g. whitepaper)
• document generalised operation on key cube operations (e.g. cube.regrid, cube.collapsed)
• document specific behaviours (e.g. on regridding + interpolation scheme classes)

I made a separate ticket, because we are still deferring the choice of whether we "release" ancillary variables in Iris 3.0 So we may want to keep documentation in separate PRs for now, on the launch_ancils feature branch.

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Note: There are virtually no statements anywhere about what happens to cell-measures. This could be improved at the same time, where appropriate.