Docs: make all API reference examples self-contained

Some code examples for functions & methods (etc.) in the API reference of the documentation start from a pre-defined state where required fields have already been setup by unspecified code, e.g. (from here) the cf.Bounds.has_property example begins with an undefined f of nature irrelevant:

>>> f.set_property('project', 'CMIP7')
>>> f.has_property('project')
True

whereas the remainder are 'sefl-contained' in that they can be executed directly without error in Python, assuming only an appropriate environment & having run import cf, e.g (from here) where d is defined before it is used for illustration:

>>> d = cf.Data([[4, 2, 1], [1, 2, 3]], 'metre')
>>> d.unique()
<CF Data: [1, 2, 3, 4] metre>

We should convert those in the former state to the latter state by specifying required setup steps. Not only would it make it more useful to users so they can explore the examples more readily, it is a prerequisite to adding docstring example checking (see #57).

Preferably, fields etc. of the required nature would be created using cf construction methods, to promote those. Failing that, they can be created &/or adapted form one of the cf.example_field field constructs, & as part of this work it may be useful to extend the number of those for variety to minimise such example setup.