edot doc: add Next.js example

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):

```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.