The patching guidelines doc needs an update

[carlossanlop]

https://github.com/dotnet/source-build/blob/main/Documentation/patching-guidelines.md

It still mentions steps involving dotnet/installer but that repo is no longer useful. I am not sure what the replacement steps should be for the sdk repo instead (or this repo).

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[MichaelSimons]

For .NET 9+, the patches would be checked into the sdk repo. When updating the document, it should have versioned guidance added as the document is intended to serve all in-support versions. e.g. for .NET 8.0 do this, for .NET 9.0+ do this.