Improve "Develop AI agents on Apify" page

[marcel-rbro]

Summary

The "Develop AI agents on Apify" page (merged via PR #2375) needs further iteration and improvements.

As noted during review, the page currently reads like unrelated things stitched together (AI agent templates, OpenRouter, AI Sandbox). The content needs better cohesion and flow.

Improvements to consider

• Improve narrative flow between sections (templates, OpenRouter, AI Sandbox)
• Add more context and explanations connecting the three main topics
• Refine structure so the page tells a coherent story
• Address any additional feedback gathered post-release

Context

• PR docs: add Develop AI agents on Apify page #2375 was intentionally shipped as a first version to iterate on
• Jiri Spilka noted the page reads like unrelated things stitched together
• Dušan Vystrčil agreed to release first, iterate afterwards