doc: restructure and reorganize docs

[bassosimone]

Create a dedicated CONTRIBUTING.md as a landing point for someone that wants to contribute to the codebase.

Cross reference design documents.

Explain the high-level data flow and how the major components in the codebase tie together.

Update code example and architecture docs.

Add deepwiki badge.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.91%. Comparing base (6238c7d) to head (de227f7).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #143   +/-   ##
=======================================
  Coverage   99.91%   99.91%           
=======================================
  Files          52       52           
  Lines        3729     3729           
=======================================
  Hits         3726     3726           
  Misses          3        3           

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