bpf, docs: Reformat BPF maps page to be more readable

Add a more complete introduction, with links to man pages.
Move toctree of map types above usage notes.
Format usage notes to improve readability.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20221012152715.25073-1-donald.hunter@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>