docs: Restructure documentation with separate introduction section

[patrikbraborec]

Summary

Re-applies the documentation restructuring from #740 (which was reverted in #763).

• Split the monolithic overview page into three focused pages: Introduction, Installation, and Quick start
• Reorganized documentation structure by moving webserver guide from concepts to guides section
• Renumbered concept pages to maintain proper ordering after moving webserver content
• Updated sidebar configuration to create a dedicated Introduction category
• Updated redirect on landing page to point to /docs/introduction

Test plan

• Verify all internal links work correctly
• Check that the sidebar navigation displays properly
• Confirm all code examples and images are accessible
• Review the new three-page structure for logical flow

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.84%. Comparing base (6034c49) to head (c3d57af).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #771      +/-   ##
==========================================
- Coverage   81.91%   81.84%   -0.08%     
==========================================
  Files          46       46              
  Lines        2693     2693              
==========================================
- Hits         2206     2204       -2     
- Misses        487      489       +2     

Flag	Coverage Δ
integration	55.95% <ø> (ø)
unit	67.99% <ø> (-0.08%)	⬇️

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[patrikbraborec]

@TC-MO I did the same changes as in JS SDK (structure-wise). Please check it.