docs: Add schema validation feature guide

[Copilot]

No dedicated guide existed explaining how schema validation works in Dataframely — only the quickstart touched on it incidentally.

Changes

• docs/guides/features/schema-validation.md — New feature guide covering:
  • Column declarations and column-level rules (nullable, primary_key, type-specific constraints, check)
  • Schema-level rules via @dy.rule() (row-level and group-level)
  • Schema inheritance
  • Inspecting a schema via repr()/print()
• docs/guides/features/index.md — Adds schema-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 (4de3bb5).

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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