Sidebar should not be floating on mobile (#1622)

When sidebar is set to 100% width on mobile, it should also be switched
back to being non-floating.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

Author: kartben

sphinx_rtd_theme/static/css/theme.css

@@ -615,3 +615,5 @@
   .rst-content
     .sidebar
       width: 100%
+      float: none
+      margin-left: 0