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>

Documentation/dev-tools/tools.rst

@@ -14,3 +14,4 @@ whole; patches welcome!
 .. toctree::
    :maxdepth: 2
 
+   coccinelle

MAINTAINERS

@@ -3124,7 +3124,7 @@ L:	cocci@systeme.lip6.fr (moderated for non-subscribers)
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/mmarek/kbuild.git misc
 W:	http://coccinelle.lip6.fr/
 S:	Supported
-F:	Documentation/coccinelle.txt
+F:	Documentation/dev-tools/coccinelle.rst
 F:	scripts/coccinelle/
 F:	scripts/coccicheck