Fix docs build + enable on PRs

[pp-mo]

As for Iris, so we get checks on the docs build via RTD

Should also do a doctest check,

~We already seem to be getting that (cf #482) "somehow", but I'm not sure how.~ (Update : pytest docs testing is configured here )

[pp-mo]

N.B. pytest doctests --> Python standard-library doctest module. Sphinx interpretation is probably the same , maybe not totally ?

[trexfeathers]

N.B. pytest doctests --> Python standard-library doctest module. Sphinx interpretation is probably the same , maybe not totally ?

https://github.com/SciTools/iris/issues/6110