docs: add agent via api docs

[tobySolutions]

Why?

This adds documentation for the API to add AI agents with subscription.

Contribution checklist?

• The commit messages are detailed
• The build command runs locally
• Assets or static content are linked and stored in the project
• Document filename is named after the slug
• You've reviewed spelling using a grammar checker
• For documentation, guides or references, you've tested the commands and steps
• You've done enough research before writing

Security checklist?

• Sensitive data has been identified and is being protected properly
• Injection has been prevented (parameterized queries, no eval or system calls)
• The Components are escaping output (to prevent XSS)

[changeset-bot]

⚠️ No Changeset found

Latest commit: 8e6a43c

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