docs: add guide on validating Actor input with Pydantic

[vdusek]

Adds a new 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 scraping-library guides.

What's included

• docs/03_guides/10_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 Pydantic features (HttpUrl/EmailStr, model_validator, SecretStr).
• docs/03_guides/code/10_pydantic.py — the runnable example Actor, shown via RunnableCodeBlock (Run on Apify) like the other guides. It validates the input, logs the normalized values, and stores them as OUTPUT.
• Cross-link to the new guide added to the Actor input concepts page and to the quick-start Guides list.

Verification

• The example Actor was run locally for both valid and invalid input: valid input passes validation and writes OUTPUT (exit 0); invalid input logs a per-field summary (errors referenced by their input-schema aliases) and the run ends FAILED.
• ruff check, ruff format --check, and ty check pass.

Numbered 10 to sit after the in-progress uv (#932, 08) and Scrapling (#938, 09) guides.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.87%. Comparing base (3f25d4a) to head (a27cf5a).

Additional details and impacted files

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

Flag	Coverage Δ
e2e	37.76% <ø> (ø)
integration	59.10% <ø> (ø)
unit	75.62% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.