[Submodule] Issues for submoduling infrastructure

- [ ] #15 Global Usings files should be replaced with props file versions to minimize needing to include files in each project, more transparent. for instance, this is a problem currently:

https://github.com/CommunityToolkit/Labs-Windows/blob/3ab75baf6a3246cf9a1891488dbcb52189a64c7e/Windows.Toolkit.Common.props#L33-L35

- [ ] Find a better home for certain things like this:

https://github.com/CommunityToolkit/Labs-Windows/blob/3ab75baf6a3246cf9a1891488dbcb52189a64c7e/Directory.Build.targets#L21-L26

- [ ] In fact do we need the base props/target files since we have our projects include a common file as a starting point, can we just move all these things (or most of them elsewhere?) - At least maybe for props. Like we should probably just have the Directory.Build.props be the common project info like we have in `Windows.Toolkit.Common.props`.

- [ ] Document which things need to exist for the sub modules to work (like `RepositoryDirectory` definition)