docs: Move kprobes.rst from staging/ to trace/

Kprobes contitutes a dynamic tracing technology and as such can be
moved alongside documentation of other tracing technologies.

Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Link: https://lore.kernel.org/r/8a09c01c64b2e003d5486c4f0c89740618f1bae4.1595331824.git.naveen.n.rao@linux.vnet.ibm.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>