Rewrite custom file input

- Changes the wrapping label to a div so we can style the label instead of another element while also supporting form validation.
- Fixes form validation styles for custom file input (closes #24831).
- Updates docs with validation styles (also adding example feedback text while I was there) and new how it works section.

scss/_variables.scss

@@ -509,12 +509,7 @@ $custom-file-box-shadow:            $input-box-shadow !default;
 $custom-file-button-color:          $custom-file-color !default;
 $custom-file-button-bg:             $input-group-addon-bg !default;
 $custom-file-text: (
-  placeholder: (
-    en: "Choose file..."
-  ),
-  button-label: (
-    en: "Browse"
-  )
+  en: "Browse"
 ) !default;