After migrating to openstackdocstheme, some shade document became
not easy to read. This commit fixes them a bit.
- openstackdocstheme assumee only one title per page.
Use a different level of title for the title.
Otherwise, titles with the same level are not shown.
- Release notes page has a lot of sections. It leads to a long TOC
in the user guide index page.
Use maxdepth=1 explicitly for the release notes.
- Add a link to a simple example to usage.rst.
It helps users who access the user guide directly.
Change-Id: If51afa471505296b502bed3288cc9bcf30a69ba3
262061afd0