[Bug] docfx should tolerate missing metadata references for project references

[AArnott]

docfx seems to really dislike when there is a project reference without a compiler reference assembly. It emits warnings like this and a failing exit code:

warning: [Warning] Found project reference without a matching metadata reference: E:\src\vs-mef\src\Microsoft.VisualStudio.Composition.Analyzers.CodeFixes\Microsoft.VisualStudio.Composition.Analyzers.CodeFixes.csproj

But there are legit cases for having a reference to a project that does not offer a reference assembly. For example when I reference a project that produces a C# analyzer. I don't want to reference that in my own compilation, but referencing it as a project reference is required to get it to show up as a nuget package dependency.

Please offer a way to suppress this warning.

Context (please complete the following information):

• OS: Windows 11
• Docfx version: 2.78.5

[hiSandog]

Analyzer project references are a good case to distinguish from a genuinely missing compiler reference. Docfx could inspect MSBuild metadata such as ReferenceOutputAssembly=false or OutputItemType=Analyzer and avoid the warning for references intentionally excluded from compilation, while retaining it for ordinary project references whose metadata assembly disappeared. A targeted suppression option is still useful, but recognizing those standard analyzer semantics would prevent every consuming repository from maintaining the same warning filter.