feat: add toolkit create reasoning tools

[omeraplak]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://voltagent.dev/docs/community/contributing/#commit-convention

Bugs / Features

• Related issue(s) linked
• Tests for the changes have been added
• Docs have been added / updated
• Changesets have been added https://voltagent.dev/docs/community/contributing/#creating-a-changeset

What is the current behavior?

Currently, adding reasoning tools (think, analyze) and associated instructions to an agent requires manual setup. The documentation provides a basic overview but could be more detailed regarding implementation and best practices.

What is the new behavior?

This PR introduces the createReasoningTools helper function, which simplifies adding reasoning capabilities. It returns a pre-configured Toolkit containing the think and analyze tools, along with optional instructions and few-shot examples for the agent's system prompt.
The documentation (reasoning-tool.mdx) has been significantly updated to:

• Explain the createReasoningTools helper and its options.
• Clarify the purpose and internal workflow of the think and analyze tools.
• Include key instructions and guidelines provided to the agent.
• Provide a more detailed usage example and expanded best practices, including linking to the Developer Console for monitoring.

fixes #24

Notes for reviewers

The main goal here is to improve the developer experience for adding and using structured reasoning in VoltAgent. The createReasoningTools helper abstracts away the setup complexity, and the updated documentation provides clearer guidance.

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