Clarify that auth restricts MCP server generation

[ethanpalm]

Documentation changes

This adds a note to https://mintlify.com/docs/mcp about how authentication restricts MCP server generation. Also makes some general copyedits to the page.

Closes https://linear.app/mintlify/issue/DOC-76/add-a-callout-to-the-mcp-page-on-the-dashboard-that-enabling-auth

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For Reviewers

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• Code examples work as written
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[baell]

lgtm