docs: bpf: Rename README.rst to index.rst

Recently bpf/ docs were converted to use RST format.  'README.rst' was
created but in order to fit in with the Sphinx build system this file
should be named 'index.rst'.  Rename file, fixes to integrate into
Sphinx build system in following patches.

docs: Rename Documentation/bpf/README.rst to Documentation/bpf/index.rst
Signed-off-by: NTobin C. Harding <me@tobin.cc>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>