Rename source directory of .NET document

[FionaDyalog]

Currently, the .NET Framework Interface document is under a directory called dotnet-interface.

This should be renamed to dotnet-framework-interface. It is the documentation specifically pertaining to the Dyalog for Microsoft Windows .NET Framework Interface. We also have a cross-platform .NET Interface (https://docs.dyalog.com/latest/dotNET%20Interface%20Guide.pdf), and that should be in a dotnet-interface directory! See issue #61 for addition of the .NET Interface Guide.