docs: add description for derived_references_self error

[FrankFMY]

Summary

Closes #17481

Adds documentation for the derived_references_self runtime error, which previously had no description on the error page.

Covers:

• What the error means (a $derived reading itself during evaluation)
• Direct self-reference example
• Indirect cycle example (two deriveds referencing each other)
• How to fix it: break the cycle by using $state and updating explicitly

This is a revised version of #17629, with corrected examples and a valid fix suggestion.

[changeset-bot]

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-17646-svelte.vercel.app/

[Rich-Harris]

marking draft until #17666 is resolved.

If we do end up just fleshing out the docs, then we really need more realistic examples — these ones don't make much sense to be honest. The one with count and total suggests that total will be updated whenever count changes, but that's not what's implied by the version with add. Both examples would be disallowed by TypeScript and thus very unlikely to arise organically:

It would be good to understand the situations in which people are encountering this.