bpftool/docs: add btf sub-command documentation

Document usage and sample output format for `btf dump` sub-command.

Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Alexei Starovoitov <ast@fb.com>
Cc: Yonghong Song <yhs@fb.com>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: Song Liu <songliubraving@fb.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/bpf/bpftool/Documentation/bpftool-perf.rst

@@ -85,4 +85,5 @@ SEE ALSO
 	**bpftool-map**\ (8),
 	**bpftool-cgroup**\ (8),
 	**bpftool-feature**\ (8),
-	**bpftool-net**\ (8)
+	**bpftool-net**\ (8),
+	**bpftool-btf**\ (8)