Enable running doctests with page build

[matthew-brett]

It is inefficient that a careful author has to first run the doctests on the project:

make doctest

then run the page build

make html

where all code has to be run twice, once for the doctests and once for the page build. Consider:

• having the doctests build the relevant outputs so that the page build does not have to rerun the nbplot etc code - see Proper caching of built nbplots #2 .
• having the doctests run as part of the page build, perhaps as a sphinx configuration option.

@stefanv - we discussed this a while ago.