Corrected Mintlify installation and launch commands

[0x0v4]

• Corrected the commands in README for installing and running Mintlify, based on the official Mintlify documentation
• Tested, everything works correctly

[cursor]

PR Summary

Updates local dev instructions to use the current Mintlify CLI.

• In README.md, replaces npm install -g mintlify with npm i -g mint and mintlify dev with mint dev

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