We need to document the ProjectCapability Items that the .NET SDK (and other SDKs in this repo) provide

### Is your feature request related to a problem? Please describe.

`ProjectCapability` MSBuild Items are declarative data that IDEs and CLI experiences (like SDK Container publishing and `dotnet run`) use to determine if a specific action or feature can be used by a given project. This set is documented somewhat at https://github.com/microsoft/vsprojectsystem/blob/master/doc/overview/project_capabilities.md, but we've certainly added more over time. We should have a place to document these items, what they mean, and what experiences (IDE and CLI) they enable.

### Describe the solution you'd like

We need two things:
* new in-repo documentation for the project capabilities provided in the .NET SDK, with separate documentation files for each 'MSBuild SDK' that provides capabilities. This gets us 'green' with the current state.
* a new github actions workflow that detects when a PR includes a new ProjectCapability and reminds the author to add/update the existing in-repo documentation. This keeps us 'green' for the future.