Currently, Ryu-Documentation page adopts 'haiku' for Sphinx
html_theme, but 'haiku' does not have page-index and no sidebar.
This patch changes Sphinx html_theme to 'sphinx_rtd_theme', which
outputs page-index for each page and a mobile-friendly sphinx
theme that was made for readthedocs.org.
Signed-off-by: IWASE Yusuke <iwase.yusuke0@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>