docs: expand rules reference with signatures and per-rule TOC

[fain182]

What

Expands docs/rules.md so it is self-contained and easier to consult (both for humans searching for a rule and for LLM coding agents generating a phparkitect.php).

For every rule:

• a one-line description of when it raises a violation
• the constructor signature (read from src/Expression/ForClasses/, not from memory)
• documentation for the previously opaque optional arguments — the $exclude allow-lists on DependsOnlyOnTheseNamespaces / NotDependsOnTheseNamespaces and $externalDependenciesToExclude on NotHaveDependencyOutsideNamespace

Also:

• a new Imports section with the base use statements and the rule that every expression lives under Arkitect\Expression\ForClasses (the file had zero imports before, so a reader landing only on this page couldn't write working code)
• a two-level table of contents linking every individual rule (anchors verified against the headings)

The existing examples are preserved.

Refs #626

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.25%. Comparing base (ea0897d) to head (9b46b83).
⚠️ Report is 15 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #634   +/-   ##
=========================================
  Coverage     98.25%   98.25%           
  Complexity      700      700           
=========================================
  Files            87       87           
  Lines          2000     2000           
=========================================
  Hits           1965     1965           
  Misses           35       35           

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