docs: Restore "docs: Added 3-month and 6-month subscription periods"

See first comment for doc stage location.

This restores commit 29b498bb , which
had been submitted prematurely, and was reverted by
commit 5c83c761

bug: 24168299

Change-Id: Ib941b459c99581217df7b5438fb1b06686e5be7d