Add official guide for cross-compiling for Linux targets

There is an existing blog post that describes how to use Qt VS Tools to cross-compile for a Raspberry Pi 3:

• https://www.qt.io/blog/creating-an-embedded-qt-quick-application-in-visual-studio-1/2
• https://www.qt.io/blog/creating-an-embedded-qt-quick-application-in-visual-studio-2/2

It would be good to include this information in the official docs at https://doc.qt.io/qtvstools/ where it is more visible to users and where it can be updated for future releases if any relevant changes occur.

A customer reported trying to follow the blog post, but had to add the following to make it work:

• Create symlinks to Qt tools in the /bin folder to the executables in the /libexec folder so they could be found
• Ensure compiler version was set to C++17 (this was not done by the wizard automatically even though Qt 6 requires this)
• Ensure compilers are set to gcc and g++ for compiling, and g++ for linking