DEV: Document 'uploads' property in theme settings (#83)

Add description for 'uploads' property in theme settings.

Should we update this document, mentioning that type: objects already
work with site settings(and plugins)?

We mentioned at the beginning of this doc that we are planning to remove
json_schemas

Author: Grubba27

docs/05-themes-components/10-objects-for-theme-settings.md

@@ -66,6 +66,7 @@ The above schema definition states that the `link` object has a `name` property
 - `integer`: Value of property is stored as an integer.
 - `float`: Value of property is stored as a float.
 - `boolean`: Value of property is `true` or `false`.
+- `uploads`: Value of property is the attachment URL
 - `enum`: Value of property must be one of the values defined in the `choices` keyword.
   ```yaml
   links: