style/standardize readme cli name

[Earnwithalee7890]

What changed? Why?
Updated "Mint CLI" to "Mintlify CLI" in the root README.md to use the official product name.
While the npm package is named mint, the official product branding is "Mintlify CLI". Using the correct product name reduces confusion for new contributors and aligns with Mintlify's official documentation.

Notes to reviewers

• Changed two instances in the "Local development" and "How to contribute" sections.
• The command npm i -g mint remains unchanged as it's the correct package name.
• No functional changes, purely a clarity and branding improvement.

How has it been tested?

• Manual review of README content.
• Verified that installation instructions and commands remain accurate.
• Confirmed terminology aligns with Mintlify's official documentation at mintlify.com.

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/2	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 1 Sum 2