fix: clarify empty .NET API diagnostics

[vicancy]

Summary

Fixes #10733.

• Keep the empty managed-reference result a warning, but make it actionable: check that inputs contain APIs to document, review API visibility and filtering rules, and include the actual configured filter path when present.
• If no assembly loaded successfully, point to input files and earlier load/compilation diagnostics instead of implying that filtering caused the empty result.
• Document the troubleshooting steps without recommending that users indiscriminately disable filters or expose private APIs.

Validation

Added 10 regression cases covering custom filters, internal-only APIs, empty sources, default filtering, <exclude />, assembly identification, unmatched inputs, compilation errors, and successful public/private-member generation.

20 targeted tests passed on .NET 10 (new diagnostics cases plus existing API filtering and assembly metadata tests):

dotnet test test/Docfx.Dotnet.Tests/Docfx.Dotnet.Tests.csproj --no-restore -f net10.0 -p:TargetFrameworks=net10.0 --filter "FullyQualifiedName~DotnetApiCatalogTest|FullyQualifiedName~ApiFilterUnitTest|FullyQualifiedName~GenerateMetadataFromAssemblyTest"

No filtering, visibility, or metadata generation defaults are changed.