Roslyn components multi-targeting needs to be documented

The work done in https://github.com/dotnet/sdk/pull/20793 should be documented. Points that should be covered:

- Why/when multi-targeting is needed.
- How to include different versions in both cases when SDK supports components multi-targeting (`SupportsRoslynComponentVersioning` is true) and when it's false.
- How to handle Roslyn APIs that exists in a newer version but not an older one.

The article can go in https://github.com/dotnet/docs/tree/main/docs/csharp/roslyn-sdk and be linked from source generators article and analyzer/codefix article.

cc @eerhardt in case you want to work on the documentation.