🍱(frontend) Docs icon overridable from theme

Depend the theme, you can have different Docs icons
in the header.
A customization was already possible from the
theme customization file, but now it is as
well possible to override the icon from the theme
itself, making it easier to manage different themes
with different icons.
We change the theme customization variable name
to "icon" instead of "logo", "logo" was already
used for the main logo of the application inside
the theme configuration.

src/helm/env.d/dev/configuration/theme/demo.json

@@ -135,7 +135,7 @@
     }
   },
   "header": {
-    "logo": {
+    "icon": {
       "src": "/assets/icon-docs.svg",
       "width": "32px"
     }