docs: record Pygments 2.20 exclusion ADR

[DecisionNerd]

Closes #547

Summary

• Add ADR 0001 documenting why GraphForge intentionally excludes Pygments 2.20.x from docs dependencies.
• Link the ADR from the Development docs navigation.
• Capture the revisit criteria before relaxing the Pygments upper bound again.

Validation

• make pre-push
• uv run --group docs mkdocs build --strict

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✨ Finishing Touches

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• Commit unit tests in branch docs/547-pygments-220-adr

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.81%. Comparing base (caaffe2) to head (815177d).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #548   +/-   ##
=======================================
  Coverage   83.81%   83.81%           
=======================================
  Files          47       47           
  Lines       12647    12647           
  Branches     3587     3587           
=======================================
  Hits        10600    10600           
  Misses       1265     1265           
  Partials      782      782           

Flag	Coverage Δ
full-coverage	83.81% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
parser	92.93% <ø> (ø)
planner	79.90% <ø> (ø)
executor	75.46% <ø> (ø)
storage	98.68% <ø> (ø)
ast	97.51% <ø> (ø)
types	90.66% <ø> (ø)

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