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: NJonathan Corbet <corbet@lwn.net>

Documentation/dev-tools/tools.rst

@@ -21,3 +21,4 @@ whole; patches welcome!
    kasan
    ubsan
    kmemleak
+   kmemcheck

MAINTAINERS

@@ -6803,7 +6803,7 @@ KMEMCHECK
 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