docs: add diagrams of core architecture

[keiya01]

Overview

What I've done

What I haven't done

How I tested

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Memo

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[codecov]

Codecov Report

Merging #605 (16a8ea6) into main (15fec35) will increase coverage by 0.00%.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #605   +/-   ##
=======================================
  Coverage   27.14%   27.14%           
=======================================
  Files         752      753    +1     
  Lines       87829    87864   +35     
  Branches     1916     1916           
=======================================
+ Hits        23844    23854   +10     
- Misses      63891    63916   +25     
  Partials       94       94           

see 4 files with indirect coverage changes