Add Public API Docs

[tembo]

Summary

Added comprehensive documentation for Tembo's Public API, including:

• Getting started guide
• SDK installation instructions
• Detailed API endpoint descriptions
• Authentication and usage examples
• Rate limits and error handling

Key additions:

• Created new features/public-api.mdx file
• Updated docs.json to include public API in navigation
• Provided code samples for task creation, repository listing, and API interactions

The documentation covers core API functionalities like:

• Creating tasks
• Listing repositories
• Authentication
• SDK usage
• Error handling and rate limits

Want me to make any changes? Add a review or comment with @tembo and i'll get back to work!

[tembo]

Requesting review from @ryw who has experience with the following files modified in this PR:

• docs.json