Sphinx reported warnings on missing implementation notes documentations in the
table of contents:

Documentation/bpf/clang-notes.rst: WARNING: document isn't included in any toctree
Documentation/bpf/linux-notes.rst: WARNING: document isn't included in any toctree

Add these documentations to the table of contents (index.rst) of BPF
documentation to fix the warnings.

Link: https://lore.kernel.org/linux-doc/202210020749.yfgDZbRL-lkp@intel.com/
Fixes: 6c7aaffb ("bpf, docs: Move Clang notes to a separate file")
Fixes: 6166da0a ("bpf, docs: Move legacy packet instructions to a separate file")
Reported-by: Nkernel test robot <lkp@intel.com>
Signed-off-by: NBagas Sanjaya <bagasdotme@gmail.com>
Link: https://lore.kernel.org/r/20221002032022.24693-1-bagasdotme@gmail.comSigned-off-by: NAlexei Starovoitov <ast@kernel.org>