feat: enhances rich text upload with custom field API

* feat: adds admin.upload.collections[collection-name].fields to the RTE to save specific data on upload elements

* chore: renames flatten to unflatten in reduceFieldsToValues, disables automatic arrow function return in eslint

* docs: adds documentation for upload.collections[collection-name].fields feature

* feat: adds recursion to richText field to populate relationship and upload nested fields

* chore: removes unused css

* fix: import path for createRichTextRelationshipPromise

* docs: updates docs to include images for the RTE upload docs

docs/fields/rich-text.mdx

@@ -77,6 +77,16 @@ The default `leaves` available in Payload are:
 
 Set this property to `true` to hide this field's gutter within the admin panel. The field gutter is rendered as a vertical line and padding, but often if this field is nested within a Group, Block, or Array, you may want to hide the gutter.
 
+**`upload.collections[collection-name].fields`**
+
+This allows [fields](/docs/fields/overview) to be saved as meta data on an upload field inside the Rich Text Editor. When this is present, the fields will render inside a modal that can be opened by clicking the "edit" button on the upload element.
+
+![RichText upload element](https://payloadcms.com/images/fields/richText/rte-upload-element.jpg)
+*RichText field using the upload element*
+
+![RichText upload element modal](https://payloadcms.com/images/fields/richText/rte-upload-fields-modal.jpg)
+*RichText upload element modal displaying fields from the config*
+
 ### Relationship element
 
 The built-in `relationship` element is a powerful way to reference other Documents directly within your Rich Text editor.
@@ -143,7 +153,7 @@ Custom `Leaf` objects follow a similar pattern but require you to define the `Le
             ]
           }
         ],
-        elements: [
+        leaves: [
           'bold',
           'italic',
           {
@@ -154,7 +164,17 @@ Custom `Leaf` objects follow a similar pattern but require you to define the `Le
               // any plugins that are required by this leaf go here
             ]
           }
-        ]
+        ],
+        upload: {
+          collections: {
+            media: {
+              fields: [
+                // any fields that you would like to save
+                // on an upload element in the `media` collection
+              ],
+            },
+          },
+        },
       }
     }
   ]