Fix Modal previews being overlapped by MkDocs header (#471)

Install the latest version of docoff that no longer establishes a new stacking context within previews.

src/docs/_overrides/main.html

@@ -22,7 +22,7 @@
   <script src="/docs/_assets/js/ruiSwatch.js" type="application/javascript"></script>
 
   <!-- Then load and init Docoff -->
-  <script crossorigin src="https://unpkg.com/@react-ui-org/[email protected].3/public/generated/bundle.js"></script>
+  <script crossorigin src="https://unpkg.com/@react-ui-org/[email protected].4/public/generated/bundle.js"></script>
 
   <!-- Then load non-Docoff code highlighiting -->
   <script src="https://unpkg.com/[email protected]/components/prism-core.min.js"></script>