Add `AddAudioSource` trait and improve `Decodable` docs

[dis-da-mor]

Objective

• Fixes #6361
• Fixes #6362
• Fixes #6364

Solution

• Added an example for creating a custom Decodable type
• Clarified the documentation on Decodable
• Added an AddAudioSource trait and implemented it for App

[harudagondi]

Reviewing this tomorrow. Also you should put each issue in a separate bullet point and prepend "Fixes" =)

[james7132]

bors r+

[bors[bot]]

Pull request successfully merged into main.

Build succeeded:

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• build (macos-latest)
• build (ubuntu-latest)
• build-wasm
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• check-compiles
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• run-examples
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