Documentation: tracing: Add ring-buffer mapping

It is now possible to mmap() a ring-buffer to stream its content. Add
some documentation and a code example.

Link: https://lore.kernel.org/linux-trace-kernel/20240510140435.3550353-5-vdonnefort@google.com

Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>