Create really simple samples for different platforms and providers

For two reasons:
* To provide a simple starting point for getting-started
* To provide a starting point for issue reports--see https://github.com/dotnet/efcore/issues/20029

For the second point, we should also update the issue template.

Matrix:
- Platforms
- [x] Console
- [x] Blazor
- [x] ASP.NET Core
- [x] Windows Forms
- [x] WPF
- [ ] UWP/WinUI
- [x] Xamarin
- Database providers
- [ ] SQL Server
- [x] SQLite
- [ ] In-memory
- [x] Cosmos
- [ ] Npgsql
- [ ] MySQL

(There is a lot of overlap here with other issues; we can de-dupe or not as appropriate as we make progress based on whether we also need more complex examples and also for creating the docs that reference the samples.)