bpftool: Add gen subcommand manpage

Add bpftool-gen.rst describing skeleton on the high level. Also include
a small, but complete, example BPF app (BPF side, userspace side, generated
skeleton) in example section to demonstrate skeleton API and its usage.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20191218052552.2915188-4-andriin@fb.com

tools/bpf/bpftool/Documentation/bpftool.rst

@@ -81,4 +81,5 @@ SEE ALSO
 	**bpftool-feature**\ (8),
 	**bpftool-net**\ (8),
 	**bpftool-perf**\ (8),
-	**bpftool-btf**\ (8)
+	**bpftool-btf**\ (8),
+	**bpftool-gen**\ (8),