bpf, docs: Move linux-notes.rst to root bpf docs tree

In commit 4d496be9 ("bpf,docs: Create new standardization
subdirectory"), I added a standardization/ directory to the BPF
documentation, which will contain the docs that will be standardized
as part of the effort with the IETF.

I included linux-notes.rst in that directory, but I shouldn't have. It
doesn't contain anything that will be standardized. Let's move it back
to Documentation/bpf.

Signed-off-by: David Vernet <void@manifault.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230828155948.123405-2-void@manifault.com