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

In commit 4d496be9ca05 ("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 <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

Documentation/bpf/index.rst

···
    bpf_licensing
    test_debug
    clang-notes
+   linux-notes
    other
    redirect
 

Documentation/bpf/standardization/index.rst

···
    :maxdepth: 1
 
    instruction-set
-   linux-notes
 
 .. Links:
 .. _IETF BPF Working Group: https://datatracker.ietf.org/wg/bpf/about/