edot doc: add Next.js example

[trentm]

We should have examples/nextjs and/or Next.js doc section in the EDOT Node.js docs.
Jessica has an example we can use or crib from.
IIUC, the core of it is using Next.js automatic loading of instrumentation.{ts,js} (https://nextjs.org/docs/app/building-your-application/optimizing/instrumentation) to start the SDK. Something roughly like:

instrumentation.ts or src/instrumentation.ts (or .js):

export async function register() {
  if (process.env.NEXT_RUNTIME === 'nodejs') {
      await import('@elastic/opentelemetry-node');
  }
}

Compare to similar usage of Vercel's own @vercel/otel described at: https://nextjs.org/docs/app/building-your-application/optimizing/open-telemetry
I'm curious whether the instrumentation compares favourably. E.g. does the Next.js by default send spans with route info to any registered global TracerProvider? Etc.

[JessicaGarson]

I went ahead and made this repository public: https://github.com/JessicaGarson/otel-nextjs-elastic/