Update discover.md with autogen-ext-mcp community package

[richard-gyiko]

Why are these changes needed?

Added a link to the extension doc for easy discovery of the library enables AutoGen agents to use tools via Model Context Protocol.

Related issue number

• N/A

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.