feat: adds showSaveDraftButton option to show draft button with autosave enabled (#12150)

This adds a new `showSaveDraftButton` option to the
`versions.drafts.autosave` config for collections and globals.

By default, the "Save as draft" button is hidden when autosave is
enabled. This new option allows the button to remain visible for manual
saves while autosave is active.

Also updates the admin UI logic to conditionally render the button when
this flag is set, and updates the documentation with an example usage.

docs/versions/autosave.mdx

@@ -22,6 +22,7 @@ Collections and Globals both support the same options for configuring autosave.
 | Drafts Autosave Options | Description                                                                                                                                                           |
 | ----------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
 | `interval`              | Define an `interval` in milliseconds to automatically save progress while documents are edited. Document updates are "debounced" at this interval. Defaults to `800`. |
+| `showSaveDraftButton`   | Set this to `true` to show the "Save as draft" button even while autosave is enabled. Defaults to `false`.                                                            |
 
 **Example config with versions, drafts, and autosave enabled:**
 
@@ -50,9 +51,13 @@ export const Pages: CollectionConfig = {
     drafts: {
       autosave: true,
 
-      // Alternatively, you can specify an `interval`:
+      // Alternatively, you can specify an object to customize autosave:
       // autosave: {
+      // Define how often the document should be autosaved (in milliseconds)
       //   interval: 1500,
+      //
+      // Show the "Save as draft" button even while autosave is enabled
+      //   showSaveDraftButton: true,
       // },
     },
   },