Doc: Add tags for Qt WebKit examples

Use the manifest meta-content system to add suitable tags for Qt WebKit
examples.

This enables users to filter for them in Qt Creator Welcome mode by
typing 'webkit'.

Change-Id: Ie9b5f6ff9f455dd3d8f7c64e5f73e30ff021f208
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>