Add Cursor rules guide

[ethanpalm]

Documentation changes

This PR adds a guide on using Cursor to write your docs.

• Overview of Cursor rules and using Cursor to write your docs with Mintlify components
• Copyable example rule

Closes https://linear.app/mintlify/issue/DOC-38/cursor-guide

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✅ Technical accuracy

• Code examples work as written
• Commands and configurations are correct
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✅ Clarity and completeness

• Instructions are clear and easy to follow
• Steps are in logical order
• Nothing important is missing
• Examples help illustrate the concepts

✅ User experience

• A new user could follow these docs successfully
• Common gotchas or edge cases are addressed
• Error messages or troubleshooting guidance is helpful

[IHSten]

One 💬 but otherwise this looks good and the preview looks good!