Add pnpm back to CLI docs

[lawreka]

Documentation changes

We shipped improvements to pnpm compatibility in https://github.com/mintlify/mint/pull/3428
This PR adds the CodeGroup snippet for installing with pnpm back into the docs

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For Reviewers

When reviewing documentation PRs, please consider:

✅ Technical accuracy

• Code examples work as written
• Commands and configurations are correct
• Links resolve to the right destinations
• Prerequisites and requirements are accurate

✅ 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

[ethanpalm]

I changed the it to pnpm add -g based on our Slack conversation. Also added updated a few more places where we tell people to install mint.

Looks good to go!