Allow adding documentation/description for resource types in bicep extensions

**Is your feature request related to a problem? Please describe.**
We are developing a bicep extension, and it would be good for our extension users to be able to see the description for resource types. Right now, we can add description on each of the properties, but not the resource itself. It sounds natural to have something like that. I see that for ARM resources, IntelliSense shows a documentation link, and I see someone asked about that for custom resources too: https://github.com/Azure/bicep/issues/17429. That would work for us, but inline documentation is preferable to have everything in code itself, like with any other programming language where documentation is in the code, instead of maintaining a separate wiki.

**Describe the solution you'd like**
Ideally, when defining a new resource, I would have a way to provide the description/documentation, and then IntelliSense would show that documentation when I hover on a resource instance in a .bicep file.

The issue names no files or tests. Start by tracing how Bicep extensions define resource types and how IntelliSense produces hover documentation; done means an extension can supply a resource-level description and users see it when hovering over a resource instance in a .bicep file.

Written by the indexing model from the issue text.