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

(cherry picked from commit bac69dc)

Author: Mpdreamz

global.json

@@ -2,5 +2,6 @@
   "sdk": {
     "version": "3.0.100"
   },
-  "version": "7.3.0"
+  "version": "7.3.0",
+  "doc_current": "7.3"
 }

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
 			{