docs: tease generateStaticParams under dynamic segments (collapsed)
The 'Creating a dynamic segment' section currently shows the request-time
`params` example and links out to the API reference for everything else.
Static pre-rendering of dynamic routes only surfaces in the deep-dive,
which puts it well after a beginner has stopped reading.
Add a collapsed `<details>` block that shows the `generateStaticParams`
shape early enough to be discoverable, without bloating the main flow.
The closing note pointing at `dynamicParams: false` keeps the cross-link
to the route segment config doc.
Also drop a stray comma after `[Dynamic Segments]` in the adjacent
sentence.
Picked up from #82773.