Adds namespace docs & adds new multitasking.md deep dive (#4114)

* Added namespace docs

* Claned up FileServices

* rewrote mainloop.md to be focused on multitasking

* rewrote mainloop.md to be focused on multitasking2

* rewrote mainloop.md to be focused on multitasking3

docfx/docfx.json

@@ -42,7 +42,8 @@
         ],
         "exclude": [
           "_site/**",
-          "includes/**"
+          "includes/**",
+          "apispec/**"
         ]
       }
     ],
@@ -54,6 +55,7 @@
         ]
       }
     ],
+    "overwrite": "apispec/*.md",
     "globalMetadata": {
       "_appName": "Terminal.Gui v2",
       "_appTitle": "Terminal.Gui v2",