Expose/document "autolinkable"

[ceball]

Allows automatic link creation from names in notebook to more detailed docs. E.g. hv links NdLayout in a notebook to the NdLayout reference notebook.

[jsignell]

I think this is beyond the scope of nbsite.

[jbednar]

I'm a bit confused; as the issue was written, it sounds like autolinkable was already part of nbsite, but I don't see it anywhere in the Python nbsite module. I don't personally think this is an essential feature, but I do think it would be a nice to have someday.