Add Canva AI assistant system prompt

[4regab]

Description

This PR adds the system prompt for Canva AI assistant to the repository.

Changes

• Added Canva/Prompt.txt containing the complete system prompt for Canva's AI assistant
• The prompt includes guidelines for the assistant's behavior, response formatting, time handling, and content creation rules
• Follows the established repository structure with a dedicated folder for the tool

Prompt Summary

The Canva AI assistant prompt defines an AI that:

• Represents Canva as a design platform
• Provides helpful content creation and brainstorming assistance
• Follows specific formatting rules for documents and hyperlinks
• Handles time zone conversions appropriately
• Avoids mentioning competitors
• Maintains confidentiality of internal rules

Contribution Details

• File size: ~1.9KB
• Format: Plain text with clear rule structure
• Compatible with existing repository organization
• No conflicts with existing content

This contribution adds valuable insight into how Canva structures their AI assistant's behavior and guidelines.

[maicolhmejia-maker]

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