docs: sphinxify kmemcheck.txt and move to dev-tools

Cc: Vegard Nossum <vegardno@ifi.uio.no>
Cc: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/MAINTAINERS b/MAINTAINERS
index b235e0d..8107235 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -6803,7 +6803,7 @@
 M:	Vegard Nossum <vegardno@ifi.uio.no>
 M:	Pekka Enberg <penberg@kernel.org>
 S:	Maintained
-F:	Documentation/kmemcheck.txt
+F:	Documentation/dev-tools/kmemcheck.rst
 F:	arch/x86/include/asm/kmemcheck.h
 F:	arch/x86/mm/kmemcheck/
 F:	include/linux/kmemcheck.h