Include version and docs generation date.

[thorwhalen]

I would be nice for the user to be able to know what version the documentation pertains to, and view earlier versions as well (like python or sklearn docs do).

It's also nice for the developer to know when the docs were generated (for example, to see if a new generation came through yet). Unless a better/easier idea comes along; generation date should be included subtly (small font, gray, footer?).