docs: add AI agents guide

[vdusek]

Add a guide on hosting AI agents as Apify Actors.

Each of the five supported frameworks (LangGraph, CrewAI, PydanticAI, LlamaIndex, and Smolagents) gets a complete, runnable single-file Actor example. Every example routes its LLM calls through the Apify OpenRouter proxy, so no provider API key is needed and each one runs as a "Run on Apify" example. The guide also covers choosing a framework, connecting to an LLM, using Apify Actors as agent tools, and monetizing with pay-per-event. 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 91.69%. Comparing base (4ead0c6) to head (2fa02f9).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #995      +/-   ##
==========================================
- Coverage   91.72%   91.69%   -0.03%     
==========================================
  Files          50       50              
  Lines        3213     3215       +2     
==========================================
+ Hits         2947     2948       +1     
- Misses        266      267       +1     

Flag	Coverage Δ
e2e	35.64% <ø> (+0.04%)	⬆️
integration	57.38% <ø> (+0.02%)	⬆️
unit	83.01% <ø> (-0.03%)	⬇️

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