bpf, documentation: Add graph documentation for non-owning refs

It is difficult to intuit the semantics of owning and non-owning
references from verifier code. In order to keep the high-level details
from being lost in the mailing list, this patch adds documentation
explaining semantics and details.

The target audience of doc added in this patch is folks working on BPF
internals, as there's focus on "what should the verifier do here". Via
reorganization or copy-and-paste, much of the content can probably be
repurposed for BPF program writer audience as well.

Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Link: https://lore.kernel.org/r/20230214004017.2534011-9-davemarchevsky@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

bpf, docs: Fix ordering of bpf documentation

This commit fixes the display of the BPF documentation in the sidebar
when rendered as HTML.

Before this patch, the sidebar would render as follows for some
sections:

| BPF Documentation
|- BPF Type Format (BTF)
|- BPF Type Format (BTF)

This was due to creating a heading in index.rst followed by
a sphinx toctree, where the file referenced carries the same
title as the section heading.

To fix this I applied a pattern that has been established in other
subfolders of Documentation:

1. Re-wrote index.rst to have a single toctree
2. Split the sections out in to their own files

Additionally maps.rst and programs.rst make use of a glob pattern to
include map_* or prog_* rst files in their toctree, meaning future map
or program type documentation will be automatically included.

Signed-off-by: Dave Tucker <dave@dtucker.co.uk>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/1a1eed800e7b9dc13b458de113a489641519b0cc.1636749493.git.dave@dtucker.co.uk