Add rejectionPlan documentation from openapi

[sahilsuman933]

Description

• Added new documentation page for Tool rejection plan under the "Tools" section.
• Included an overview, detailed schema, examples for various conditions (regex, liquid, group), and usage in assistant hooks.
• Updated fern/docs.yml to link the new page in the navigation sidebar.

Testing Steps

• Run the app locally using fern docs dev or navigate to preview deployment
• Ensure that the changed pages and code snippets work

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[github-actions]

🌿 Preview your docs: https://vapi-preview-051b7bd4-abb3-417d-b930-ab1f5c44f835.docs.buildwithfern.com

[github-actions]

🌿 Preview your docs: https://vapi-preview-c44ab5cc-7982-4f54-9c31-cc64cd930e08.docs.buildwithfern.com

[github-actions]

🌿 Preview your docs: https://vapi-preview-a9b6a8b3-a138-471f-bacf-e9bc1e9c5b6d.docs.buildwithfern.com