Move EF6, EF5, and EF4.3.1 API reference to Docs

The API references for the non-core versions of the NuGet package should be part of the .NET API Browser. I talked to @dend and apparently they can be hosted in this repo.

To have complete coverage we discussed having the API reference generated for the following EntityFramework package versions:

- https://www.nuget.org/packages/EntityFramework/6.2.0
- https://www.nuget.org/packages/EntityFramework/5.0.0
- https://www.nuget.org/packages/EntityFramework/4.3.1

Current locations which need to be redirected
- EF6
- https://msdn.microsoft.com/library/dn223258(v=vs.113).aspx (our conceptual docs point here)
- https://msdn.microsoft.com/en-us/library/mt154346(v=vs.113).aspx (duplicate)

- EF5
- https://msdn.microsoft.com/en-us/library/hh289362(v=vs.103).aspx