Recently bpf docs were converted to RST format. The new files were not
added to the top level toctree. This causes build system to emit a
warning of type
WARNING: document isn't included in any toctree
Add bpf/index.rst to Documentation/index.rst
Signed-off-by: Tobin C. Harding <me@tobin.cc> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>