Add .opus audio to various docs

[Joy-less]

This pull request is based on the changes in godotengine/godot#117921 (Add support for .opus audio files).

Summary of changes:

• Adds Opus as an option when referring to WAV, MP3 and OGG Vorbis.
• Adds a recommendation to use Opus to reduce file size while maintaining quality.
• Adds a section including a comparison for the transparent bitrate for each format.
  • MP3 and Ogg Vorbis transparency bitrates are based on the Hydrogenaudio Knowledgebase.

    MP3-encoded files are generally considered artifact-free at bitrates at/above 192kbps.
    Vorbis ogg files are supposedly artifact-free at bitrates at/above 160kbps.

  • Opus transparency bitrate is based on the Audacity manual.

    Opus at 128 kbps VBR (default settings) is pretty much transparent.

  • WAV transparency bitrate is based on standard CD-quality (44.1kHz, 16-bit, stereo).

Resolved

[Calinou]

Looks good to me.

I'm a bit surprised that the estimated transparency bitrate for Opus is 128 Kb/s, as I often read it was around 96 Kb/s in other places. I guess we can trust Hydrogenaudio's advice on this still.

[Joy-less]

I'm a bit surprised that the estimated transparency bitrate for Opus is 128 Kb/s, as I often read it was around 96 Kb/s in other places. I guess we can trust Hydrogenaudio's advice on this still.

Indeed. Opus recommends 64-96kbps for music streaming, but 96-128kbps for music storage.
According to https://listening-test.coresv.net/results.htm, volunteers rated 96kbps Opus inbetween "Imperceptible" and "Perceptible, but not annoying" for every track.
Probably fine for a game, but not transparent until 128kbps.