There were two Release notes information: doc/source/releases and
releasenotes (by reno). There is no need to have two different way.
It is not a good idea to have too much contents in the top page
document. Installation stuff is split into a separate page.
Title level of index.rst was inconsistent.
_static and _templates directories in releasenotes/source
are unnecessary. Also version info is set in release notes.
Change-Id: I717964f0dc219d50037f2616e500053f769dc1d0
c5ddeb56c2