docs: Add container introspection and debugging guide

[mikelane]

Summary

• Add docs/guides/debugging.md covering the three container introspection methods: debug(), explain(), and graph()
• Add the guide to the Advanced Topics toctree in docs/index.md

What the guide covers

• Quick reference table comparing debug() vs explain() vs graph()
• debug(): registration summary, diagnosing "adapter not found" errors, checking lifecycle components, writing output to files
• explain(cls): resolution tree visualization, port resolution, wrong adapter diagnosis, circular dependency detection, unregistered type handling
• graph(): Mermaid and Graphviz DOT output formats, reading the graph, CI integration with a GitHub Actions example, embedding in documentation
• Combined debugging workflow showing how to use all three methods together

Test plan

• Sphinx build succeeds with no warnings related to the new file
• New guide appears in the Advanced Topics toctree
• Code examples match the actual API signatures in container.py
• Output examples match the format produced by the implementation

Fixes #351

Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.85%. Comparing base (9f37e0b) to head (d376dc3).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #474   +/-   ##
=======================================
  Coverage   91.85%   91.85%           
=======================================
  Files          21       21           
  Lines        1903     1903           
  Branches      372      372           
=======================================
  Hits         1748     1748           
  Misses         91       91           
  Partials       64       64           

Flag	Coverage Δ
python-3.13	91.85% <ø> (ø)

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