Doc: List more platforms on Building from Sources page

Separate building from source instructions from the getting
started documentation for Qt for iOS.

Add links to build instructions for Embedded Linux, Android, iOS,
and WinRT platforms to 'Building Qt Sources' page and update the
links on 'Qt Configure Options'.

Task-number: QTBUG-37145
Change-Id: I4e9bfbfb55dbb2f6f63e68b105987b5e587189b7
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>