docs: add client-side rendering note to beginner courses

[marcel-rbro]

Summary

• Adds an :::info admonition to the "Downloading HTML" lesson (lesson 4) in both Python and JavaScript beginner web scraping courses
• Explains that some websites use client-side rendering and the techniques in the course only work for server-rendered pages
• Links to the Puppeteer and Playwright course for handling JavaScript-rendered sites

Closes #1900

Test plan

• Verify the admonition renders correctly in both course lessons
• Verify the link to the Puppeteer and Playwright course works

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[apify-service-account]

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[marcel-rbro]

Review: Add client-side rendering note to beginner courses

Overall, this is a useful addition that sets correct expectations for beginners early in the course. The placement is logical - right after the first successful HTML download, before the HTTP protocol deep-dive. Here is a detailed review against the project style guide.

1. Admonition title - PASS

The admonition has a title ("Client-side rendering"), which is required by content-standards.md. The title is concise and scannable (2 words).

2. Writing style - MINOR ISSUES

• Imperative tone: Mostly good. The text is informational rather than instructional, which is appropriate here.
• No first person: Pass - no "I" or "we" used.
• Active voice: Pass.
• US English: Pass.

One suggestion: The last sentence starts with "See the..." which is fine, but per writing-style.md link guidance, the link text itself could be more action-oriented. Currently it reads:

See the Puppeteer and Playwright course for handling such websites.

Consider rewording to make the link text more descriptive of the action (per the "Action-oriented text" section in writing-style.md):

To handle such websites, take the Puppeteer and Playwright course.

This is a minor nit - the current version is acceptable.

3. Admonition type - PASS

:::info is appropriate here. Per the style guide, :::info is for "Background information and explanations." This admonition provides background context about a limitation of the techniques being taught. It is not a tip (actionable suggestion), note (general callout), or warning (gotcha). Good choice.

4. Link correctness - PASS

The link /academy/puppeteer-playwright is correct. The Puppeteer and Playwright course index has slug: /puppeteer-playwright and lives under the academy route base path (routeBasePath: 'academy' in the Docusaurus config). Other files in the repo reference this same path pattern (e.g., https://docs.apify.com/academy/puppeteer-playwright/...).

5. Placement - PASS

The admonition is placed right after the learner's first successful HTML download and before the "Client and server, request and response" tip. This is the ideal position because:

• The learner just saw their first HTML output and might wonder "what if the HTML looks empty?"
• It naturally flows before the HTTP theory section
• It does not interrupt the step-by-step coding flow

6. Sentence length - PASS

All sentences are well under 30 words (longest is 18 words). Good.

7. Terminology - PASS

No Apify product names are used, so no terminology violations. Generic technical terms like "JavaScript", "HTML", "browser-based approach" are used correctly in lowercase.

8. Consistency between Python and JavaScript versions - PASS

The admonition text is identical in both files, which is correct since the concept applies equally to both courses.

Summary

This PR is in good shape. One optional improvement to consider:

• Optional: Rephrase the last sentence link text to be more action-oriented (see point 2 above).

No blocking issues found. Approve when ready.