doc/_theme: customize sphinx_rtd_theme

doc/_theme: customize sphinx_rtd_theme

* move the breadcrumbs to the top
* add border around admonition elements
* use different colors and fonts for section headers
* add decoration lines at the bottom of breadcrumbs
* remove left and right borders in tables
* override the injected versions, the name of theme
  is different from "sphinx_rtd_theme", but the
  versions element is still displayed at the
  bottom-left corner as "versions.html" defines.
  without overriding .rst-badge CSS styling,
  readthedocs puts the injected versions at
  the default bottom-right corner, see
  https://github.com/readthedocs/readthedocs.org/blob/2a519f1146142d18f6a63b61c2f08984067280e0/readthedocs/api/v2/templates/restapi/footer.html

Signed-off-by: Kefu Chai <kchai@redhat.com>