Updated customise theme docs

The custom theme document talks about System Icons, then immediatly ignores the existance of these  in the `Sidebar Sub-menu` example a couple of paragraphs later.
This commit updates the docs to make use of these icons, as well as adding an icon for resource types to the default icons.

Also updated the sidebar in example app to use this same approach

Signed-off-by: Alex Crome <afscrome@users.noreply.github.com>

packages/app-defaults/src/defaults/icons.tsx

@@ -34,6 +34,7 @@ import MuiMenuBookIcon from '@material-ui/icons/MenuBook';
 import MuiPeopleIcon from '@material-ui/icons/People';
 import MuiPersonIcon from '@material-ui/icons/Person';
 import MuiWarningIcon from '@material-ui/icons/Warning';
+import MuiWorkIcon from '@material-ui/icons/Work';
 
 export const icons = {
   brokenImage: MuiBrokenImageIcon as IconComponent,
@@ -56,6 +57,7 @@ export const icons = {
   'kind:location': MuiLocationOnIcon as IconComponent,
   'kind:system': MuiCategoryIcon as IconComponent,
   'kind:user': MuiPersonIcon as IconComponent,
+  'kind:resource': MuiWorkIcon as IconComponent,
   user: MuiPersonIcon as IconComponent,
   warning: MuiWarningIcon as IconComponent,
 };