Add starting point for mobx related documentaion (#1422)

* add a brief overview to our use of mobx and mark all observable fields as observable in our docs

* add to navigation

Signed-off-by: Sebastian Malton <sebastian@malton.name>

mkdocs.yml

@@ -33,6 +33,7 @@ nav:
       - Main Extension: extensions/guides/main-extension.md
       - Renderer Extension: extensions/guides/renderer-extension.md
       - Generator: extensions/guides/generator.md
+      - Working with mobx: extensions/guides/working-with-mobx.md
     - Testing and Publishing:
       - Testing Extensions: extensions/testing-and-publishing/testing.md
       - Publishing Extensions: extensions/testing-and-publishing/publishing.md