docs(crawlee): warn against router: as top-level CheerioCrawler option

[DaveHanns]

Summary

When reviewing AI-generated Crawlee code (from ChatGPT, Cursor, Claude Code, etc.), one very common mistake is passing the router under a router option — or in addition to requestHandler:

// This does not work:
new CheerioCrawler({ router, requestHandler: async (ctx) => { /* ... */ } });

Crawlee has no top-level router option. The initializing_and_setting_up.md lesson in scraping_basics_legacy/challenge/ already shows the correct pattern (requestHandler: router), so this PR adds a short :::caution admonition immediately after that code sample which:

1. States the rule (router is passed as requestHandler, there is no router option).
2. Quotes the two error messages a reader will see verbatim (runtime ow error + TS2353), so search hits from those exact strings land here.
3. Shows the correct + incorrect snippet side-by-side.
4. Notes that this is a recurring AI-suggested pattern to watch for.

No existing code paths or examples change; the addition is a single admonition block.

Evidence

Both error messages are reproducible with any recent Crawlee release.

Runtime (ow predicate rejection at construction):

ArgumentError: Did not expect property `router` to exist, got ...
    at new HttpCrawler ...

TypeScript compile-time:

error TS2353: Object literal may only specify known properties,
and 'router' does not exist in type 'CheerioCrawlerOptions'.

Minimal reproducer:

import { CheerioCrawler, createCheerioRouter } from 'crawlee';

const router = createCheerioRouter();
router.addDefaultHandler(async ({ log }) => log.info('hi'));

// ❌ fails: no top-level `router` option
new CheerioCrawler({ router, requestHandler: async () => {} });

// ✅ works
new CheerioCrawler({ requestHandler: router });

Test plan

• Docusaurus renders the new :::caution block correctly in light + dark themes.
• The two text code blocks inside the admonition render without syntax-highlighter warnings.
• Anchors / TOC of the page are unaffected (no new heading was added).

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Surfaced during an evaluation of Apify surfaces for agent-driven Actor development.