Update documentation for links.
Update documentation for links.

- Add details about the different link scenarios
- Add an explicit explicit linking section
- Add the allowed properties for providers
- Add the allowed properties for consumers
- Add FAQ section
- Add details about `nil` providers
- Add details about `nil` consumers

[#158511327](https://www.pivotaltracker.com/story/show/158511327)

Co-authored-by: Dale Wick <188d987a4b971a1fecf708bf002a3b94c86c96a2@pivotal.io>

cloudfoundry/docs-bosh