Document how to install & register protoc on Windows

Qt Protobuf (and by extension Qt GRPC) require an installed protoc compiler. Anyhow, the current documentation just gives some general advise on Linux how to get protobuf, and does not give any hints for Windows. Also the linked upstream documentation does not mention Windows.

We should therefore suggest a) where to get protoc from , and b) how to tell CMake about it. What seems to work as of now is:

• Download Windows binaries from Githubs Protobuf Releases Page, extract to some directory.
• Adding <install_dir>\bin to CMAKE_PREFIX_PATH environment variable.
• Run CMake

This should be documented. Also, I think the individual examples like Magic8Ball should prominently mention this requirement.