doc: Sphinxify the tracepoint docbook

Convert the tracepoint docbook template to RST and add it to the core-api
manual.  No changes to the actual text beyond the mechanical formatting
conversion.

Cc: Jason Baron <jbaron@redhat.com>
Cc: William Cohen <wcohen@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>