Ensure docs still build (2 man days)

[DPeterK]

Ensure the docs still build, fixing any problems

Doc build checks:

• "main" documentation
• gallery
• doctests

[DPeterK]

Building docs for iris/master (@ 538eed4): build succeeded, 100 warnings.

Building docs for iris/dask (@ 88db1d2): build succeeded, 100 warnings.

The good news is the docs still build and the number of docs-build warnings has not changed. I haven't checked yet whether the warnings being produced are the same or not. I'll do that soon...

Of course, almost all of the documentation that we've added is in private modules (I'm looking at you, _lazy_data) so this result shouldn't be much of a surprise.

[DPeterK]

Completed by #2576