Add entries for cppgraphqlgen to the C/C++ section

[wravery]

Description

Both of these libraries/tools are part of the microsoft/cppgraphqlgen project, but I split them into separate entries to make it clear that it supports generating both client and server implementations in C++.

The markdown looks fine to me in a previewer, but I haven't been able to get the site to run the "Code" page locally (with or without my addition) to verify these changes look OK on the site. It throws an error trying to dereference an undefined data variable on my machine. If there are any problems with the formatting or the way this looks on a working build, please feel free to send it back for fixes.

Thanks!