API guide: register MCP server

[ntrogh]

Fixes #8216

[isidorn]

Assigning @digitarald and @connor4312 as reviewers first

[connor4312]

I would not merge this until we finalize an API, as it's not very useful for extensions until then and is still subject to change

[ntrogh]

Thanks. I'll hold off on merging this.

[ntrogh]

TODO: incorporate API updates from microsoft/vscode-extension-samples#1182

[connor4312]

The MCP extension API is now finalized so we should be good to go here :)

Final version: https://github.com/microsoft/vscode/pull/248244/files#diff-0a75aed19c118603eb96332bc0b9c2d7867f4182346d16d18b7fc31b6ceeb321

[ntrogh]

@connor4312 I've updated the API guide with the latest API changes. Ready for your review.

[ntrogh]

@connor4312 Thanks for the pointer to the resolve method. Added description and example. Ready for your review.