Validate C# code samples in docs

The docs now contain a lot of C# code samples (thanks for that, Stephen and Daniel). Unfortunately, some of these samples don't use proper C# syntax and therefore won't compile.

Since I have already noticed in the past that some listings don't work any more with more recent versions of Gremlin.Net, we should not only correct these listings, but actually validate them automatically to ensure that we notice once the listings don't work any more.

I limited the scope to C# here because I already noticed problems there, but this could of course also be done later for the other GLVs that can't be verified with the usual doc generation process.