Improve NFC documentation

1.) Extend the NFC overview page
2.) Ensure that snippets are compiled at all times
3.) Mark Qt 5.3 APIs using \since

Task-number: QTBUG-32401
Task-number: QTBUG-34978
Change-Id: I8928be3d7dd2c9de1314eb1ec6ed55c8d4301f4a
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>
Fix code snippet error in docs and point out supported NFC platforms

The code snippet was shifted into a compiled project to ensure continued
compilation.

Task-number: QTBUG-34978
Change-Id: I93f3b39442992bfb4049a44ed0dd6330d3ca9270
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>