Document *.server.* and $lib/server functionality

[elliott-with-the-longest-name-on-github]

Describe the problem

I wrote #6422 and the more recent #6623. I suggested some documentation in the first PR, but we weren't sure where to put it. We need to add the latest feature to that documentation and actually add it to the docs.

Describe the proposed solution

Do the thing.

Alternatives considered

Scrap SvelteKit and start over

Importance

i cannot use SvelteKit without it

Additional Information

No response

[accuser]

Some documentation for the $lib/server functionality should probably be included with Project Files / src and Modules / $lib.

Looking at the current documentation I don't think I'd be sure where the *.server.* functionality should sit either.