Update urcu page

- add paper ref,
- remove "why on lttng website"
- remove details about debian/arch. If we explain specifics of some
  distro, we should do it for all distros.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Merge pull request #1 from compudj/gh-pages

Update urcu page