bpf, docs: Generate nicer tables for instruction encodings

Use RST tables that are nicely readable both in plain ascii as well as
in html to render the instruction encodings, and add a few subheadings
to better structure the text.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211223101906.977624-4-hch@lst.de