Better documentation for saving hybrid heights

Nobody has claimed this yet.

# 📚 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?).

Start by checking the CF conventions and the iris _pp_save_rules.py reference mentioned in the issue. Document the requirements for saving netCDF files and F03 ancillaries with hybrid heights, including the underspecified-coordinate failure mode and whether orography is required on load; confirm the details against those references.

Written by the indexing model from the issue text.