tools lib traceevent: Introduce man pages

Initial support for libtraceevent man pages - Documentation directory,
templates, configurations, Makefiles.

The first man page is also part of the patch - summary of the library
and all its APIs.

Building of the documentation is integrated into the libtraceevent build
process, new targets are added to its Makefile:

make help
make doc
make doc-clean
make doc-install
make doc-uninstall

Signed-off-by: Tzvetomir Stoyanov <tstoyanov@vmware.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: linux-trace-devel@vger.kernel.org
Link: http://lore.kernel.org/linux-trace-devel/20190503091119.23399-2-tstoyanov@vmware.com
Link: http://lkml.kernel.org/r/20190510200106.104812629@goodmis.org


[ Replaced tracefs tracing/events to tracefs events in DESCRIPTION section ]
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>