Link referenced APIs from Activity API index

I often land on the Activity API index and then have to hunt for a link to one of the APIs in the sidebar. We should just link to the APIs in the page copy.

![screen shot 2013-11-13 at 4 43 49 pm](https://f.cloud.github.com/assets/865/1536602/2228b9c4-4cb5-11e3-864a-3b46171eccb1.png)


I'd be happy to add a CSS change to avoid the blue headings if desired.

@gjtorikian @tobiasahlin