doxy: remove custom HTML div sections before doxygen sectons

They were only added to match the "runtime" css style, which is not
used anymore. The current doxygen-generated HTML is invalid because
it embeds those div declarations in <p>...</p>. Also anchors were
duplicated to move them a bit up.

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>