docs/kbuild/makefiles: drop section numbering, use references

Drop the manually updated section numbering. It's hard to maintain, and
indeed hasn't been updated when sections 3.4 and 7.8 were dropped.

Update all the section references to rst references.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>