Merge branch 'bpf-docs-rst-improvements'

Tobin C. Harding says:

====================
Recently BPF docs were converted to RST format. A couple of things were
missed:

 - Use 'index.rst' instead of 'README.rst'.  Although README.rst will
   work just fine it is more typical to keep the subdirectory indices
   in a file called 'index.rst'.

 - Integrate files Documentation/bpf/*.rst into build system using
   toctree in Documentation/bpf/index.rst

 - Include bpf/index in top level toctree so bpf is indexed in the main
   kernel docs.

 - Make anal change to heading format (inline with rest of Documentation/).
====================

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>