Add note about preloading to routing introduction.

[leerob]

Preloading is mentioned in the next/link docs, but not in the routing introduction.

[stigkj]

@leerob This change seems to say that prefetching is not done for SSR pages. While the documentation for next/link seems to not be in agreement with this. Or is it just my understanding that is off?? ;-)

[leerob]

Correct - which part needs updated there? I can tackle that 👍

[stigkj]

I'm not sure what it is supposed to say, because I am not sure what actually happens 😛 Is it prefetching SSR pages? And if that is the case, how (or what)?

[leerob]

SSR routes aren't prefetched, as added in this PR. The next/link docs say this - which part needs more clarity to you?

[stigkj]

I think it should be more explicit, like it is in this PR. I.e. say that SSR pages are not prefetched. As it is now I thought that SSR pages were prefetched, while static pages did some extra stuff for even faster prefetching.

[stigkj]

And where are the discussion around why SSR pages are not prefetched?

[timneutkens]

The JS for SSR pages is prefetched. Just not the data.

[stigkj]

Ok 👍 What is the reason for not prefetching data for SSR pages? Not that it needs to be the default, but having it as an option would be nice.