docs: add guide for writing custom rules

[fain182]

What

Adds docs/custom-rules.md, documenting how to extend PHPArkitect by writing your own Expression — a first-class path, not a workaround.

It covers:

• the Expression interface (describe() / evaluate(), plus the optional appliesTo())
• the ClassDescription accessors available to query a class, with their real return types
• an annotated walkthrough of the built-in IsFinal as a minimal template
• a from-scratch example (HaveAtMostDependencies, a coupling budget) showing a constructor argument and querying dependencies
• how to use a custom expression in a config (and as a selector in that())
• how to test it

The two code examples were executed against the real Arkitect classes to confirm they compile and behave as documented.

The README links to the new page so the extension path is discoverable.

Refs #626

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.25%. Comparing base (ea0897d) to head (869c37f).
⚠️ Report is 15 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #635   +/-   ##
=========================================
  Coverage     98.25%   98.25%           
  Complexity      700      700           
=========================================
  Files            87       87           
  Lines          2000     2000           
=========================================
  Hits           1965     1965           
  Misses           35       35           

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