Add behavioral audio/video recordings example

[bendichter]

This PR adds a new example dataset: beh_audio_video_recordings/.

It demonstrates the proposed audio/video behavioral recording support (see bids-standard/bids-specification PR #2231) including:

• _audio.* and _video.* recordings in sub-*/beh/
• corresponding sidecar JSON files
• multi-camera recordings using the recording entity
• audio runs using the run entity
• _events.tsv/_events.json annotations for speech/vocalization tasks
• *_scans.tsv for acquisition timing alignment

Note: media files are small placeholder files for example purposes.

[robertoostenveld]

I don't think the README.source.md should be there, right?

[bendichter]

I don't think the README.source.md should be there, right?

@robertoostenveld, Good catch. I fixed it.