Roslyn components multi-targeting needs to be documented

[Youssef1313]

The work done in dotnet/sdk#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.

[BillWagner]

Thanks for adding this @Youssef1313

I've added it to our backlog.

[Youssef1313]

For reference: a commit that has moved to multi-targeting is YairHalberstadt/stronginject@40c2721

Tagging @jnm2 in case you wanted to contribute the article :)

[jnm2]

One thing I haven't done yet but want to try is multitargeting Roslyn v2 and v3 in one package. That requires a .targets that's a bit more involved than "If not SupportsRoslynComponentVersioning then delete roslyn4."