tracing.7: Add a single reference point for tracing facilities in FreeBSD

FreeBSD has a fair number of tracing facilities. The new tracing(7)
manual page aims to provide a starting point for users to learn about
what is available.

Reviewed by: christos, bnovkov, markj, ziaee
Approved by: christos (mentor), bnovkov (mentor), markj (mentor)
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D50854