bpf, docs: Add abi.rst document to standardization subdirectory

bpf, docs: Add abi.rst document to standardization subdirectory

As specified in the IETF BPF charter, the BPF working group has plans to
add one or more informational documents that recommend conventions and
guidelines for producing portable BPF program binaries. The
instruction-set.rst document currently contains a "Registers and calling
convention" subsection which dictates a calling convention that belongs
in an ABI document, rather than an instruction set document. Let's move
it to a new abi.rst document so we can clean it up. The abi.rst document
will of course be significantly changed and expanded upon over time. For
now, it's really just a placeholder which will contain ABI-specific
language that doesn't belong in other documents.

Signed-off-by: David Vernet <void@manifault.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230828155948.123405-3-void@manifault.com