Align to SciTools/iris documentation standard

[bjlittle]

Align the Sphinx documentation look'n feel, configuration and building to that of iris.

See SciTools/iris#3752

[bjlittle]

Ping @tkknight

[pp-mo]

After quite a wait on this, we still don't seem to have a consistent Scitools style.
Think we can close for now @pp-mo @trexfeathers

[bjlittle]

@pp-mo I think this is still applicable.

To make it more concrete, the target is to adopt documentation patterns and infrastructure akin to SciTools/iris

Perhaps @tkknight has a list of top-level features that he'd like to see adopted and prioritised, given that he's been curating the documentation over on SciTools/iris ?