protobuf and grpc installation example should be clearer

<Issue>
I followed the installation example in the documentaiton(https://doc.qt.io/qt-6/qtprotobuf-installation-windows-vcpkg.html) but it's failing. The documentation should be clearer so that other users can just follow what's writen to complete the installation.

<Reproduction Step>
1. Open cmd on Windows 11

2. Execute "C:\Program Files\Microsoft Visual Studio\2022\Professional\VC\Auxiliary\Build\vcvars64.bat" to set up the environment variables

3. Change directories to C:\Program Files\Microsoft Visual Studio\2022\Professional\VC\vcpkg

4. Execute  
vcpkg.exe install protobuf protobuf:x64-windows
<Expected result>
protobuf gets installed.

<Actual result> 
The error message shows as follows:

C:\Program Files\Microsoft Visual Studio\2022\Professional\VC\vcpkg>vcpkg.exe install protobuf protobuf:x64-windows
error: Could not locate a manifest (vcpkg.json) above the current working directory.
This vcpkg distribution does not have a classic mode instance.