Update documentation for WSL2 support

What does this Merge Request do and why?

Adds additional information on known issues and configurations for getting the GDK working on Windows Subsystem for Linux.

Related gitlab#347422 (closed)

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.