Fix links in docs for unmaintained releases

Links in docs for unmaintained branches (Yoga and Zed) are still
pointing to stable/<release> branches in repository. Instead they
should point to unmaintained/<release>. This patch aims to fix that.

Closes-Bug: #2066052

Change-Id: I7e30ee5754ca2ecf9d4a2d3519f9e0bdf3f0aeea