libperf: Add man pages

Change the man page generation to asciidoc, because it's easier to use
and has been more commonly used in related projects. Remove the current
rst pages.

Add 3 man pages to have a base for more additions:

libperf.3 - overall description
libperf-counting.7 - counting basics explained on simple example
libperf-sampling.7 - sampling basics explained on simple example

The plan is to add more man pages to cover the basic API.

The build generates html and man pages:

$ cd tools/lib/perf/Documentation
$ make
ASCIIDOC libperf.xml
XMLTO libperf.3
ASCIIDOC libperf-counting.xml
XMLTO libperf-counting.7
ASCIIDOC libperf-sampling.xml
XMLTO libperf-sampling.7
ASCIIDOC libperf.html
ASCIIDOC libperf-counting.html
ASCIIDOC libperf-sampling.html

Add the following install targets:

install-man - man pages
install-html - html version of man pages
install-examples - examples mentioned in the man pages

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20191206210612.8676-3-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>