Summary:
We had rendering support for prev links, but we never had any previous links in our metadata. Only next links. This adds that support to both Guides and APIs.
**For guides**: `previous` is manually inserted into the metadata of the actual markdown file.
**For APIs/Components**: `previous` is established via code within `extractDocs.js`
> This isn't totally perfect. For example, the transition from the last guide to the first API/component has a next link from the guide, but not a previous link from the API since the way you get the previous links are different from guides and APIs. But this gets us really close.
Closes https://github.com/facebook/react-native/pull/8754
Differential Revision: D3557972
Pulled By: hramos
fbshipit-source-id: e270bb51e7a4f59f61dad28ae0928d27d0af3d4a
Eric Sauter
Joel Marcey
Vesa Laakso
Kevin Lacker
