docs: Reference svelte/reactivity from $state.

[brunnerh]

Make it easier to find SvelteSet, SvelteMap, etc.

Is there a way to also link to documentation/docs/98-reference/21-svelte-reactivity.md?

Svelte 5 rewrite

Please note that the Svelte codebase is currently being rewritten for Svelte 5. Changes should target Svelte 5, which lives on the default branch (main).

If your PR concerns Svelte 4 (including updates to svelte.dev.docs), please ensure the base branch is svelte-4 and not main.

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint
  ^ still broken for me

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[benmccann]

@brunnerh it looks like there's a merge conflict in this PR. could you update it? thanks!