docs: Add configuration injection guide

[mikelane]

Summary

• Add docs/guides/configuration.md covering config injection patterns: Pydantic Settings integration, multiple config classes, testing with register_instance(), environment-specific config, startup validation, and common mistakes
• Add register_instance() API reference section within the guide
• Update README "Common Patterns" section to use the preferred @service config pattern instead of the older ConfigPort adapter pattern
• Add configuration guide to the docs toctree under Advanced Topics

Test plan

• Sphinx build succeeds with no new warnings
• All code examples use current dioxide API (Container(profile=...), @service, register_instance())
• No references to deprecated patterns (container.scan() without profile keyword)
• Verify links resolve on ReadTheDocs after deploy

Fixes #348

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 (3d70057).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #472   +/-   ##
=======================================
  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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