docs: rework dynamic segments intro section (#94126)
We don't refer to these as `path params` anymore, rather called them
`dynamic segments`, which causes a drop-off when people search for
"nextjs path params".
While looking at this, I also noticed that we don't really mention what
a segment, people don't know this necessarily.
We'll have to come back to this point later on because, params are just
placeholders, the dynamic qualifier is too broad, dynamic at build and
runtime is what we mean today, but arguably params known via
generateStaticParams, are no longer dynamic params, they are known set
of params.
