fix: update docfx.json for DocFX v2 compatibility

- Add DocFxForUnity.csproj (netstandard2.1) and unignore it: DocFX v2
  uses MSBuildWorkspace when given a .csproj instead of raw .cs globs
- Add allowCompilationErrors: true so DocFX continues despite missing
  UnityEngine assembly references
- Add disableGitFeatures: true to avoid git-related failures in CI
- Replace retired xref.docs.microsoft.com xrefService with the new
  learn.microsoft.com .NET xrefmap
- Keep UnityXrefMaps xref for Unity API cross-references

https://claude.ai/code/session_01HDAawqjLMudEktivP5ifXw

Author: claude

.gitignore

@@ -31,6 +31,7 @@ Packages/*
 ExportedObj/
 .consulo/
 *.csproj
+!DocFxForUnity.csproj
 *.unityproj
 *.sln
 *.suo

DocFxForUnity.csproj

@@ -0,0 +1,5 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>netstandard2.1</TargetFramework>
+  </PropertyGroup>
+</Project>

Documentation/docfx.json

@@ -5,16 +5,15 @@
                 {
                     "src": "..",
                     "files": [
-                        "Assets/**/*.cs"
-                    ],
-                    "exclude": [
-                        "Assets/Plugins/**"
+                        "DocFxForUnity.csproj"
                     ]
                 }
             ],
             "globalNamespaceId": "Global",
             "filter": "filterConfig.yml",
-            "dest": "api"
+            "dest": "api",
+            "allowCompilationErrors": true,
+            "disableGitFeatures": true
         }
     ],
     "build": {
@@ -71,11 +70,9 @@
             }
         },
         "xref": [
-            "https://normanderwan.github.io/UnityXrefMaps/xrefmap.yml"
-        ],
-        "xrefService": [
-            "https://xref.docs.microsoft.com/query?uid={uid}"
+            "https://normanderwan.github.io/UnityXrefMaps/xrefmap.yml",
+            "https://learn.microsoft.com/en-us/dotnet/.xrefmap.json"
         ],
         "dest": "../_site"
     }
-}
+}