docs: add description for derived_references_self error

[benjosua]

Resolves #17481

Adds documentation for the derived_references_self runtime error.

Addresses feedback from #17646 and #17629: uses a realistic example (high score tracking) as suggested in #17666, fixes the error example to use var to avoid a TypeScript false positive on the error line, and notes that the error can occur indirectly via circular component bindings.

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-17879-svelte.vercel.app/

[Rich-Harris]

Thank you. Whether we add this will depend on whether we decide to do #17666