remove the "coming soon" sections of the documentation

When I was looking for some docs to point to, I found there are several sections there that say "coming soon" or are much thinner than the headline implies.

No info:
http://docs.couchbase.com/developer/dotnet-2.0/performance-tuning.html
http://docs.couchbase.com/developer/dotnet-2.0/managing-clusters.html

No useful info:
http://docs.couchbase.com/developer/dotnet-2.0/handling-errors.html (doesn't really cover handling errors)

I don't see any reason to have a navigation element for something we don't have any info for. We can add it when we add the info. It looks a bit amateurish to just have "Coming soon!", in my opinion.