Docs: improve custom tool guide

[lusmoura]

AI Description

This PR updates the documentation for creating custom tools, specifically focusing on Langchain tools. It provides detailed guidance on implementing custom tools, including two illustrative examples:

• Calculator Tool: This tool evaluates mathematical expressions. It includes a call() method, which receives a dictionary of parameters and returns a list of dictionary results with a "result" field.
• Python Interpreter Tool: This tool executes arbitrary Python code against a Python interpreter. It demonstrates the usage of a secret interpreter_url set in the environment variables.

Changes:

• Renames the "Implementing a Tool" section to "Implementing a Langchain Tool" for clarity.
• Adds a new section, "Implementing a custom Tool", providing detailed instructions and examples for creating custom tools.
• Includes two illustrative examples, the Calculator Tool and the Python Interpreter Tool, showcasing the implementation of the call() method and parameter handling.

[codecov-commenter]

Codecov Report

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[tianjing-li]

Approved!