Doc: Cleaned up the supported platforms section

- Removed the compiler notes page as it was only pointing
to the supported platforms page.
- Added links to pages providing information about the
Mac OS X developer environment and platform notes.

Task-number: QTBUG-30173
Change-Id: Ic2ac5365055858240a77fe3235243e4e8a584c78
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>