Add Documentation for the XML Extension TypeScript API

[datho7561]

In Extensions.md, as of PR #340, we reference a specific merged PR for documentation on the TypeScript API for extending LemMinX.

I think it would be a good idea to update Extensions.md in order to better explain the API, and link to the documentation of the API.

[angelozerr]

@ntotten @a2975667 is there any chance that you could provide a PR for that?