Better documentation for saving hybrid heights

[hdyson]

📚 Documentation Request

Addition

We should document the requirements for saving files with hybrid height coordinates. This should include:

1. What is needed to save a netCDF with CF compliant hybrid heights? (Maybe worth referencing cf checker?). This can be confirmed with reference to CF conventions.
2. What is needed to save an F03 ancillary with hybrid height? In particular, this should include the failure mode for underspecified hybrid height coordinates (i.e. if iris does not have sufficient information, it silently drops the vertical coordinate information, so the resulting ancillary has N surface fields rather than N fields on N vertical levels). The full requirement here can be confirmed by checking the iris pp_save_rules.py:

For an outline, I think the requirements for both 1 and 2 are (but these should be confirmed!):

1. DimCoord with standard name "model level number", units of "1", attribute "positive" with value "up", and points corresponding to the model levels (i.e. 1 to N for N levels).
2. AuxCoord with long name "sigma", units of "1", with both points and bounds, representing the terrain following component of the hybrid height
3. AuxCoord with long name "level height", units of "m", with both points and bounds, representing the height above surface in the absense of orography.

Note the orography is not required for save. But is required on load (this may be worth documenting too?).

[hdyson]

Bonus points for including that iris, CF conventions, F03 main text, and F03 appendix A all use different terminology. Extra bonus points for mapping those terminologies together.