Clarify Bun README preload and build-plugin requirements

[nicohrubec]

Update packages/bun/README.md with a separate instrumentation file and a bun --preload ./instrument.js app.js startup example.

Briefly explain that preload ensures initialization order, while sentryBunPlugin from @sentry/bun/plugin enables build-time instrumentation of supported bundled dependencies. Dependencies needing this instrumentation must be included in the bundle. Link to the full build example rather than duplicating it.

Preload already existed before v11; the build-plugin explanation is the key v11 addition.

Docs PR: getsentry/sentry-docs#19361

[linear-code]

JS-3738