feat: Add theme/config-point capability and docs. (#2641)

This is the initial addition of the config point settings to allow run time loading of GUI settings via "theme" files/parameters.
The intent is to add an example PR that shows when to use config point, how to configure it, write documentation for it etc.

platform/viewer/public/theme/theme.json5

@@ -0,0 +1,3 @@
+{
+  // Currently empty because nothing is configurable yet with themes
+}