docs: Add Browser Use guide

[vdusek]

Adds a guide for the Browser Use library — an LLM agent that drives a real browser — in Apify Actors, following the structure of the existing scraping-library guides.

• docs/03_guides/09_browser_use.mdx — the guide: introduction & features, configuring the LLM (OpenAI / Anthropic / Google), a runnable example Actor, Apify Proxy integration, and running on the Apify Playwright base image.
• code/09_browser_use.py — runnable single-file example: a Browser Use agent that returns structured data (Hacker News top posts) through Apify Proxy.
• Quick-start guides list updated.

Verified locally (apify run) and on the platform (build + run SUCCEEDED) — the agent navigated through Apify Proxy and produced correct structured output. Lint + type-check pass.

Closes: #836

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.94%. Comparing base (3f25d4a) to head (c4580b8).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #943      +/-   ##
==========================================
+ Coverage   86.87%   86.94%   +0.06%     
==========================================
  Files          48       48              
  Lines        2942     2942              
==========================================
+ Hits         2556     2558       +2     
+ Misses        386      384       -2     

Flag	Coverage Δ
e2e	37.76% <ø> (ø)
integration	59.10% <ø> (ø)
unit	75.69% <ø> (+0.06%)	⬆️

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