docs: Add guide on validating Actor input with Pydantic

[vdusek]

Adds a guide on validating Actor input with Pydantic, so Actor code works with a typed, guaranteed-valid object instead of reaching into a raw input dictionary. Follows the structure of the existing guides.

• docs/03_guides/11_pydantic.mdx — the guide: why raw-dict access is fragile, defining an input model (aliases bridging camelCase↔snake_case, defaults, constraints, a custom validator, extra='ignore'), validating with model_validate and failing fast on ValidationError, the relationship to the platform input schema, and a few extra features (HttpUrl/EmailStr, model_validator, SecretStr).
• code/11_pydantic.py — the runnable example Actor (Run on Apify), plus the 11_http_url.py, 11_model_validator.py, and 11_raw_input.py snippets.
• Cross-links added to the Actor input concepts page and the quick-start guides list.

Verified locally for valid input (passes validation, writes OUTPUT, exit 0) and invalid input (per-field summary keyed by input-schema alias, run ends FAILED). Lint + type-check pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.94%. Comparing base (3f25d4a) to head (bd52c61).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #941      +/-   ##
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+ Coverage   86.87%   86.94%   +0.06%     
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  Files          48       48              
  Lines        2942     2942              
==========================================
+ Hits         2556     2558       +2     
+ Misses        386      384       -2     

Flag	Coverage Δ
e2e	37.76% <ø> (ø)
integration	59.10% <ø> (ø)
unit	75.69% <ø> (+0.06%)	⬆️

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