DocFX bundles roslyn rather than using the version from the SDK

Related https://github.com/dotnet/docfx/issues/10808

We noticed this when running against projects targeting .NET 10.0. They cannot use new language features, and starting with RC2 they stopped loading the source-generators built into the framework.

I found this was because docfx was using a copy of roslyn bundled inside it. Instead it should be either a) locating the roslyn from the SDK used by the project or b) allowing the user to specify a path as an escape hatch.