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.

TODO before merging

• Clone the guide content (docs/03_guides/11_pydantic.mdx + its docs/03_guides/code/11_*.py snippets) into website/versioned_docs/version-3.4/ so it also shows in the current docs version, not only under "next".

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.70%. Comparing base (a87e8d1) to head (e8a0dea).
⚠️ Report is 1 commits behind head on master.

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HEAD has 1 upload less than BASE

Flag	BASE (a87e8d1)	HEAD (e8a0dea)
integration	1	0

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #941       +/-   ##
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- Coverage   90.00%   78.70%   -11.31%     
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  Files          49       49               
  Lines        3053     3053               
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- Hits         2748     2403      -345     
- Misses        305      650      +345     

Flag	Coverage Δ
e2e	35.66% <ø> (ø)
integration	?
unit	78.70% <ø> (ø)

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[Mantisus]

LGTM. Just a few suggestions