Align Node README initialization with v11 #24554

Update packages/node/README.md to show preloading the instrumentation file with node --import for both CommonJS and ESM applications. Correct the implication that a top-level import alone provides equivalent automatic instrumentation, and make filenames consistent.

Keep the existing bundling guidance and link to the installation guide for alternative setups.

Docs PR: getsentry/sentry-docs#19368