chore(py): Update docs for 0.5.2 release

[huangjeff5]

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[gemini-code-assist]

Code Review

This pull request updates the Genkit Python documentation across several modules, including RAG, tool calling, interrupts, and various integrations such as Vertex AI and Google GenAI. Key changes include transitioning to ModelConfig, simplifying the RAG documentation to focus on core abstractions, and adding details on context caching and per-request configurations. Feedback identifies several technical errors in the documentation examples: Part objects are incorrectly re-wrapped in Part(root=...) in multiple places, the Document object's text property is mistakenly called as a method, and there is a typo in the thought_tokens attribute name.