Show link to v0.8 release notes

They already existed, but were intentionally hidden from the list of
release notes prior to the release of 0.7 to avoid confusion.

docs/source/releases/index.rst

     :maxdepth: 1
 
     v0.7
+
+0.8 release branch
+
+.. toctree::
+    :maxdepth: 1
+
+    v0.8