docs: Retitle and streamline the existing guides

[vdusek]

Improves the existing framework guides — clearer titles, flatter example code, and a few content fixes. The id: slugs and URLs are unchanged, so existing links keep working.

• Retitle guides 01–06 to action-oriented names (e.g. "Use Crawlee" → "Building crawlers with Crawlee").
• Flatten the scraper examples (BeautifulSoup, Parsel, Playwright, Selenium): extract a scrape_page helper and track crawl depth via Crawlee's built-in Request.crawl_depth instead of a manual user_data counter. Each stays a single file with one runnable "Run on Apify" block.
• Crawlee & Scrapy guides: tidy the Apify Proxy wording, and fix the Scrapy __main__.py tab label/title plus a grammar nit.
• Web server guide: fix the Actor.configuration.web_server_url/web_server_port references (the prose used non-existent container_* attributes), add a FastAPI example and an Actor Standby section, and renumber it 07 → 12 to make room for the new guides.
• Quick-start: refresh the guides list.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.91%. Comparing base (3f25d4a) to head (10c8199).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #939      +/-   ##
==========================================
+ Coverage   86.87%   86.91%   +0.03%     
==========================================
  Files          48       48              
  Lines        2942     2942              
==========================================
+ Hits         2556     2557       +1     
+ Misses        386      385       -1     

Flag	Coverage Δ
e2e	?
integration	59.14% <ø> (+0.03%)	⬆️
unit	75.62% <ø> (ø)

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