Update readme with official Sinch Mailjet MCP server implementation

[obennett-m]

Description

Server Details

N/A

Motivation and Context

Adds to list of official MCP servers.

How Has This Been Tested?

Tested Sinch Mailjet MCP server with Claude Desktop and Warp 2 terminal.

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 Protocol Documentation
• My changes follows MCP security best practices
• I have updated the server's README accordingly
• I have tested this with an LLM client
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have documented all environment variables and configuration options

[obennett-m]

Hey @olaservo. Do I close this PR myself, or is there something else it needs to hang around for?

[olaservo]

Thanks for your contribution to the servers list. This has been merged in this combined PR: #2374

This is a new process we're trying out, so if you see any issues feel free to re-open the PR and tag me.