Complete Deno README import-hook prerequisites

[nicohrubec]

Update the existing auto-instrumentation section in packages/deno/README.md:

• Keep the supported --preload=npm:@sentry/deno/import approach.
• Document nodeModulesDir: "auto" for local dependency resolution.
• Add the permissions needed to run the example and send events.
• Distinguish permissions needed by the hook from those providing source context and system information. The docs cover network access, --allow-env, --allow-read=., and --allow-sys=hostname,osRelease; system-information permission is not required for instrumentation itself.

Link to the guide for permission details and troubleshooting. The docs PR shows a first-import approach; retaining preload is supported by the SDK's own import-hook entry point.

Docs PR: getsentry/sentry-docs#19365

[linear-code]

JS-3739