Initial user docs for pint support

[pp-mo]

If we merge the FEATURE_cfpint branch, we will have in Iris :

• an experimental control switch, iris.experimental.units.USE_CFPINT, allowing you to switch to pint units by default
• a public make_unit() call, used to create Iris unit objects from any input str/None/pint/cfpint/cf_unit

We will have a what's-new entry, but we should also have a dedicated page explaining the principles.
The basics are discussed in this discussion.

Key points to note, which the user docs should include :

• iris .units properties may be either either cf_units.Unit or pint.Unit type
• in both cases, the actual unit type is converted (on assignment) to a specialised Iris subtype with extra functionality
• unit given as strings are converted to a default type, selectable as either cf_units or pint
• Iris pint-based units
  • are pint units, and support the pint APIs
  • are based on 'cfpint' units
    • defined in cfpint repo
    • in turn based on pint, with additions :
      • translation from udunits-like strings
      • support of cftime date units, including a calendar property
  • also support the Iris-specific special units of "unknown" and "no-unit"
  • currently provide "legacy", methods mimicking cf_units, which will eventually be removed
• user code
  • can adopt pint-based units, either by supplying Pint (or cfpint) objects, or by enabling "USE_CFPINT".
  • can then use pint (and cfpint) unit APIs.

[pp-mo]

Additional points, from discussion with @trexfeathers ..

Regarding the Iris Unit classes: see the still-evolving implementation in #7027

• start with the Iris classes "CfUnit" + "PintUnit", made public within iris.experimental.units
  (i.e. API docs are available),
• these classes can actually reside in the eventual intended place, e.g. iris.common.units._pint
  we then publicise them by exporting/exposing them in iris.common.units.
• the 'compatibility' layer remains a private intermediate class "_IrisCfulikeCfPintUnit" in this inheritance chain.
  • the compatibility methods, on the "CFulike" one, will appear on the public PintUnit, so should all be marked as "deprecated" at first introduction.

We will also need some minimal readable cfpint docs for this to refer to.
See SciTools/cfpint#6

[trexfeathers]

Also need to to some tidy-up of classes that have their own units docstrings - we updated the one for CFVariableMixin but Cube has its own, and others might also