Provide a way to convert docfx map files to intersphinx inventories

[danroth27]

See dotnet/docfx#123

[agjohnson]

This seems it should be straight forward, we'll have to watch this for more information on the proposed format.

[danroth27]

I believe docfx uses a custom .rpk format. It looks like the .rpk files for the .NET Framework are published in this NuGet package: https://www.nuget.org/packages/msdn.4.5.2/0.1.0-alpha-1508031919. It looks like the .rpk format is a custom binary format, so it might actually make more sense to write a .NET tool that can do the conversion. We would need documentation though on the .inv format so we could figure out how to generate it.

[AWhetter]

Closing this now that we are dropping support for documenting languages other than Python. See #248 for more information on this decision.