Add title to index page of developer documentation

At the moment the title of the index page of the developer documentation is
"Kolla’s Mission". Other projects use "Welcome to X’s developer documentation!".

Change-Id: Iadddc7fbe3d1eeafe5ed1926a3e090b5de70e7ee

doc/index.rst

@@ -14,6 +14,10 @@
       License for the specific language governing permissions and limitations
       under the License.
 
+=================================
+Welcome to Kolla's documentation!
+=================================
+
 Kolla's Mission
 ===============