Doc: Adding a "Mobile APIs" page.

Introduces Qt's APIs for mobile platforms.

Change-Id: I079fcb81f833ad77312106849e11713efcec9961
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>
Reviewed-by: Nico Vertriest <nico.vertriest@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>