Update quickstart.mdx to remove note HTML block

[jimbobbennett]

The HTML note block doesn't render correctly in GitHub, with links inside that block broken. Changed to a block quote to fix the link.

Motivation and Context

README change to fix link styling

How Has This Been Tested?

N/A

Breaking Changes

N/A

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[jonathanhefner]

@jimbobbennett, thank you for submitting a PR, but the docs are intended to be read primarily on https://modelcontextprotocol.io/registry/ (for example, https://modelcontextprotocol.io/registry/quickstart). The <Note> component is rendered by Mintlify as a callout.

Since the docs render correctly on https://modelcontextprotocol.io/registry/, I will close.