Doc Generation move to .NET core 3.0 (#4113)

- Separate version from current doc version
- Source files should point to reference doc version as origin
- Use Json.NET to parse global.json
- Reference Buildalyzer package

For some reason the projects referenced by docs need LangVersion in the csproj file itself on linux while working fine on windows

Author: Mpdreamz

global.json

@@ -2,5 +2,6 @@
   "sdk": {
     "version": "3.0.100"
   },
-  "version": "8.0.0"
+  "version": "8.0.0",
+  "doc_current": "master"
 }

src/CodeGeneration/DocGenerator/AsciiDoc/GeneratedAsciidocVisitor.cs

@@ -73,7 +73,7 @@ public override void VisitDocument(Document document)
 				_newDocument.Attributes.Add(new AttributeEntry("nuget", "https://www.nuget.org/packages"));
 
 			var originalFile = Regex.Replace(_source.FullName.Replace("\\", "/"), @"^(.*Tests/)",
-				$"{github}/tree/{Program.BranchName}/src/Tests/Tests/");
+				$"{github}/tree/{Program.DocVersion}/src/Tests/Tests/");
 
 			_newDocument.Insert(0, new Comment
 			{