Add option for docs with single page

[drasmuss]

There isn't an easy way to test this within this repo, since everything shares the same sidebar. We'd have to build two versions of the docs and then somehow link them together. But it's a pretty minor change, and will be tested in downstream repos, so I figured that wasn't worth the effort.

Fixes #32

[hunse]

Just fixed a minor typo in the changelog (the last digit of the PR url was missing).

[hunse]

I checked the generated docs for all three pytest- repos, and they all look good (i.e. the sidebar looks as it did before). Also checked that non-onepage docs (i.e. nengo-bones docs) still look OK.