This commit adjust the sphinx configuration to use the "Read The Docs"
theme, which has the advantage of allowing the navigation bar
(containing among other things a search bar, and the TOC) to stay
fixed while scrolling the contents of the page being read. This is
particularly useful to allow access to those features while reading
a long page, for instance.
TN: VB25-026
Change-Id: Ibfebbf1d66ed34e72c4f17a90f0272ed7af787b7
(cherry picked from commit 637f02e303)
86c045ce1b
