docs: Add "How Validation Works" feature guide

[Copilot]

There was no dedicated guide explaining dataframely's validation internals — how column constraints become named rules, how @dy.rule() and group rules are assembled, and what the evaluation pipeline actually does step-by-step.

Changes

• New guide docs/guides/features/validation.md covering:

  • Column rules: how constructor arguments (nullable, min, regex, check, …) map to named Polars expressions with the <col>|<rule> naming convention
  • Schema-level @dy.rule() rules (row-level and group-level), including group_by="primary_key"
  • Null-handling semantics (null → valid by default; override with .fill_null(False))
  • The automatic primary_key uniqueness rule
  • The five-step evaluation pipeline: schema matching → rule collection → evaluation (with group-rule join mechanics) → horizontal aggregation → filter/raise
  • The cast parameter and |dtype cast-verification rules
  • All three validation entry-points (validate, filter, is_valid) with examples
  • A rule naming reference table

• docs/guides/features/index.md — added validation as the first entry in the features toctree

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (72fb1a6) to head (1156030).

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files           56        56           
  Lines         3218      3218           
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  Hits          3218      3218           

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