docs: Revert documentation restructure

[patrikbraborec]

Summary

This PR reverts commit 89c0358 which restructured the documentation by splitting the overview page into three separate pages.

Changes

• Restores the original single overview page structure
• Moves the webserver guide back from guides to concepts section
• Renumbers concept pages back to their original order
• Removes the separate introduction and installation pages

Reason for revert

[Add reason here]

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.95%. Comparing base (aa0fa9d) to head (3f35433).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #763      +/-   ##
==========================================
+ Coverage   81.91%   81.95%   +0.03%     
==========================================
  Files          46       46              
  Lines        2693     2693              
==========================================
+ Hits         2206     2207       +1     
+ Misses        487      486       -1     

Flag	Coverage Δ
integration	55.95% <ø> (ø)
unit	68.10% <ø> (+0.03%)	⬆️

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[Pijukatel]

What is the [Add reason here]?

[patrikbraborec]

@Pijukatel the PR was merged, but it still needed review by @TC-MO. It was my mistake, so we agreed that I will revert it, and we will do proper PR with all the necessary reviews. Sorry.