Add D2 diagram and add it to the README

[basnijholt]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.25%. Comparing base (75f8a6b) to head (9479e59).
⚠️ Report is 157 commits behind head on main.

Files with missing lines	Patch %	Lines
adaptive_scheduler/_scheduler/slurm.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #151   +/-   ##
=======================================
  Coverage   67.25%   67.25%           
=======================================
  Files          22       22           
  Lines        2306     2306           
=======================================
  Hits         1551     1551           
  Misses        755      755           

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