refactor: restructure documentation

[patrikbraborec]

Summary

• Restructured documentation from flat structure to numbered hierarchical organization (01_introduction, 02_concepts, 03_guides), matching the pattern used in apify-client-js#850
• Renamed 01_overview to 01_introduction with simplified overview and separate quick start page
• Renamed 03_examples to 03_guides
• Simplified overview page by removing content duplicated in the quick start guide (authentication, running Actors, providing input, getting results)
• Updated sidebar, docusaurus config, and landing page redirect

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.60%. Comparing base (a293601) to head (0cf4788).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #647      +/-   ##
==========================================
- Coverage   96.62%   96.60%   -0.03%     
==========================================
  Files          45       45              
  Lines        4271     4271              
==========================================
- Hits         4127     4126       -1     
- Misses        144      145       +1     

Flag	Coverage Δ
integration	94.70% <ø> (+0.07%)	⬆️
unit	75.60% <ø> (ø)

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