[Work Items] Drive remaining ref assembly issues for mainstream release

[jcouv]

• document and announce the feature
• project-to-project references (P2P reference treated as metadata reference when ref assemblies are enabled project-system#2478, not blocking)
• Can we improve the build log in Output window to show when compilation was skipped? (Ref assemblies: Output window should make it clear whether a project was compiled #22326, not blocking)
• Should we consider support in legacy project system after all?
• LUT (https://github.com/dotnet/testimpact/issues/1950, broken, PR https://github.com/dotnet/testimpact/pull/2589)
• lightweight solution load
• dogfood in Roslyn repo
  • Problem with excessive rebuilding FastUpToDate fails if PreserveNewer file has same timestamp as source project-system#2743 (not ref assembly-specific)
  • Problem with excessive rebuilding Don't write files in CompilerPackage if not needed #21928 (extension was updating target files)
• project-to-project patch for 15.3 (Patch up project output paths from ref assemblies when inserting P2P references #20412)
• an issue with MSBuildWorkspace (Add Project.OutputRefFilePath and ProjectInfo.OutputRefFilePath #25376)
• Should we reconsider legacy project support?
• Should ref assemblies include xml documentation? (Auto generated ref assemblies does not include xml documentations. #32215)

Test plan notes for ref assemblies: #18612

[jcouv]

All remaining possible work items are tracked as individual issues. We don't need this umbrella anymore.