De-XHRify BaseAudioContext examples

[wbamberg]

Part of #30131.

This updates the three pages under https://developer.mozilla.org/en-US/docs/Web/API/BaseAudioContext that were using XHR in examples.

This PR aligns the page with the example updates that were made in http://github.com/mdn/webaudio-examples/pull/116.

[github-actions]

Preview URLs

• /en-US/docs/Web/API/BaseAudioContext/createConvolver
• /en-US/docs/Web/API/BaseAudioContext/createScriptProcessor
• /en-US/docs/Web/API/BaseAudioContext/decodeAudioData

External URLs (4)

URL: /en-US/docs/Web/API/BaseAudioContext/createScriptProcessor
Title: BaseAudioContext: createScriptProcessor() method

• https://mdn.github.io/webaudio-examples/script-processor-node/ (1 time) (Note! This may be a new URL 👀)

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URL: /en-US/docs/Web/API/BaseAudioContext/decodeAudioData
Title: BaseAudioContext: decodeAudioData() method

• https://mdn.github.io/webaudio-examples/decode-audio-data/callback/ (1 time) (Note! This may be a new URL 👀)
• https://mdn.github.io/webaudio-examples/decode-audio-data/promise/ (1 time) (Note! This may be a new URL 👀)

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URL: /en-US/docs/Web/API/BaseAudioContext/createConvolver
Title: BaseAudioContext: createConvolver() method

• https://mdn.github.io/webaudio-examples/voice-change-o-matic/ (1 time) (Note! This may be a new URL 👀)