Add low_level public API

[samuelcolvin]

needs more docs, otherwise I think this is complete.

[hyperlint-ai]

PR Change Summary

Added a new public API for low-level functionality in the pydantic_ai module, with initial documentation provided.

• Introduced the low_level public API in the pydantic_ai module
• Added initial documentation for the new API
• Enhanced the overall API structure

Added Files

• docs/api/low_level.md

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[github-actions]

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