docs: add detailed architecture overview to README

[abekoh]

This PR adds a comprehensive architecture overview section to the README.md file. The new section includes:

• Detailed breakdown of core components
• Description of key features
• Better organization of existing content

The changes make it easier for new users to understand the system's architecture and capabilities.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 45.17%. Comparing base (bfcc809) to head (5b21335).

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #17      +/-   ##
==========================================
- Coverage   45.20%   45.17%   -0.03%     
==========================================
  Files          25       25              
  Lines        6709     6709              
==========================================
- Hits         3033     3031       -2     
- Misses       2922     2923       +1     
- Partials      754      755       +1     

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[abekoh]

This PR is built by OpenHands.