Add AI Assistant Credits Documentation

[mintlify]

This PR adds new documentation that explains AI assistant credits including:

• What a credit is equal to (1 credit = 1 AI message)
• Where credits are used in the platform (Docs side panel, Slack bot, Assistant API)
• How to track credit usage

This documentation will help users understand how AI assistant credits work and how they're consumed across different parts of the Mintlify platform.

[ethanpalm]

Closing in favor of #1073 which adds more info on assistant messages and other new features.