docs/core-api/symbol-namespaces: drop table of contents and section numbering

docs/core-api/symbol-namespaces: drop table of contents and section numbering

The manually updated table of contents and section numbering are hard
to maintain.

Make changes similar to the following commits:

  5e8f0ba38a4d ("docs/kbuild/makefiles: throw out the local table of contents")
  1a4c1c9df72e ("docs/kbuild/makefiles: drop section numbering, use references")

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>