docs: sphinxify coccinelle.txt and add it to dev-tools

No textual changes have been made, but the formatting has obviously been
tweaked.

Cc: Michal Marek <mmarek@suse.com>
Cc: Gilles Muller <Gilles.Muller@lip6.fr>
Acked-by: Nicolas Palix <nicolas.palix@imag.fr>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>