doc: Warn users about qdoc's clang dependency

From Qt 5.11, QDoc requires clang from LLVM 3.9 or later.
This new dependency must be listed in the Qt documentation.
This update adds a note to the Getting Started with Qt page
that refers the user to the QDoc manual for further instructions.

Change-Id: Ia308400c855d19b6391d9239ab74b060e931cd50
Task-number: QTBUG-65762
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>