docs: Document $lib/server and *.server.* server-only modules

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

Closes #6640.

Just documenting the PRs referenced in the issue. Added a new section for the main documentation and also added a few snippets elsewhere.

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🦋 Changeset detected

Latest commit: d2e70f4

The changes in this PR will be included in the next version bump.

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@sveltejs/kit	Patch

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[Greenheart]

Nice work! This will be very useful in a project I'm working on. Keep it up! 😄

[martinbeentjes]

@Rich-Harris this is awesome, these little improvements make using SvelteKit so good. Thank you!