docs: add AI agents guide

[vdusek]

Add a guide on hosting AI agents as Apify Actors.

It includes a runnable PydanticAI example that routes its LLM calls through the Apify OpenRouter proxy (no provider API key needed), a framework comparison table, sections on connecting to an LLM and using Apify Actors as agent tools, and short snippets for CrewAI, LangGraph, LlamaIndex, and Smolagents. Every framework links to its matching Actor template.

The new guide is also linked from the README, the overview, and the quick start.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.51%. Comparing base (726620b) to head (1bb3288).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #995      +/-   ##
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- Coverage   90.54%   90.51%   -0.04%     
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  Files          49       49              
  Lines        3132     3132              
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- Hits         2836     2835       -1     
- Misses        296      297       +1     

Flag	Coverage Δ
e2e	36.14% <ø> (ø)
integration	57.12% <ø> (-0.04%)	⬇️
unit	79.27% <ø> (ø)

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