feat(js): Update automatic instrumentation docs

[mydea]

This PR updates the browser JS "Automatic Instrumentation" docs:

1. The heartbeatTimeout option does not exist anymore, I replaced it with the new childSpanTimeout. This has been replaced in v8.
2. We used "transaction" everywhere still, I rewrote this to use the term span instead.

Closes getsentry/sentry-javascript#14805

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Name	Status	Preview	Comments	Updated (UTC)
sentry-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Dec 20, 2024 9:53am

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[codecov]

Bundle Report

Changes will decrease total bundle size by 15 bytes (-0.0%) ⬇️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
sentry-docs-server-cjs	10.36MB	9 bytes (-0.0%) ⬇️
sentry-docs-client-array-push	9.28MB	6 bytes (-0.0%) ⬇️

[coolguyzone]

Thanks for the update!