selftests/bpf: Allow skipping docs compilation

Currently rst2man is required to build bpf selftests, as the tool is
used by Makefile.docs. rst2man may be missing in some build
environments and is not essential for selftests. It makes sense to
allow user to skip building docs.

This patch adds SKIP_DOCS variable into bpf selftests Makefile that when
set to 1 allows skipping building docs, for example:
make -C tools/testing/selftests TARGETS=bpf SKIP_DOCS=1

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250510002450.365613-1-mykyta.yatsenko5@gmail.com