docs: drive: document Branding step needed to publish own client_id

Google now requires an app homepage URL and privacy policy URL to be
set on the OAuth consent screen's "Branding" page before the "PUBLISH
APP" button becomes clickable, even for a personal single-user app.
The existing instructions jumped straight to publishing in step 9
without mentioning this, leaving the button greyed out with no
explanation of why.

Fixes #9854

docs/content/drive.md

@@ -2029,6 +2029,15 @@ Scroll down and click "+ Add users". Add yourself as a test user and press save.
 9. Go to "Audience" and then click "PUBLISH APP" button and confirm.
    Add yourself as a test user if you haven't already.
 
+    (If the "PUBLISH APP" button is greyed out, Google now requires a
+    homepage URL and a privacy policy URL before publishing, even for a
+    personal single-user app. Go to "Branding" in the left panel, fill in
+    "Application home page" and "Application privacy policy link" - a free
+    GitHub Pages site is fine for these if you don't have a domain of your
+    own - then "Authorized domains" with the domain you used, and click
+    Save. Return to "Audience" and the "PUBLISH APP" button should now be
+    available.)
+
 10. Provide the noted client ID and client secret to rclone.
 
 11. Run the web-based authorization flow from within `rclone config`, by answering