Add \examplecatory to the Qt Multimedia examples

Add \examplecategory to the following examples:

1. "Audio Devices Example", https://doc-snapshots.qt.io/qt6-dev/qtmultimedia-audiodevices-example.html
2. "Audio Output Example", https://doc-snapshots.qt.io/qt6-dev/qtmultimedia-audiooutput-example.html
3. "Audio Recorder Example", https://doc-snapshots.qt.io/qt6-dev/qtmultimedia-audiorecorder-example.html
4. "Audio Source Example", https://doc-snapshots.qt.io/qt6-dev/qtmultimedia-audiosource-example.html
5. "Camera Example", https://doc-snapshots.qt.io/qt6-dev/qtmultimedia-camera-example.html
6. "Media Player Example", https://doc-snapshots.qt.io/qt6-dev/qtmultimedia-player-example.html
7. "QML Camera Application", https://doc-snapshots.qt.io/qt6-dev/qtmultimedia-declarative-camera-example.html
8. "QML Media Player Example", https://doc-snapshots.qt.io/qt6-dev/qtmultimedia-video-mediaplayer-example.html
9. "QML Video Example", https://doc-snapshots.qt.io/qt6-dev/qtmultimedia-video-qmlvideo-example.html
10. "QML Video Recorder", https://doc-snapshots.qt.io/qt6-dev/qtmultimedia-video-recorder-example.html
11. "Screen Capture Example", https://doc-snapshots.qt.io/qt6-dev/qtmultimedia-screencapture-example.html
12. "Spectrum Example", https://doc-snapshots.qt.io/qt6-dev/qtmultimedia-spectrum-example.html
13. "Video Graphics Item Example", https://doc-snapshots.qt.io/qt6-dev/qtmultimedia-videographicsitem-example.html
14. "Video Widget Example", https://doc-snapshots.qt.io/qt6-dev/qtmultimedia-videowidget-example.html