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

No textual changes beyond formatting.
Acked-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Acked-by: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/dev-tools/tools.rst

@@ -18,3 +18,4 @@ whole; patches welcome!
    sparse
    kcov
    gcov
+   kasan

MAINTAINERS

@@ -6587,7 +6587,7 @@ L:	kasan-dev@googlegroups.com
 S:	Maintained
 F:	arch/*/include/asm/kasan.h
 F:	arch/*/mm/kasan_init*
-F:	Documentation/kasan.txt
+F:	Documentation/dev-tools/kasan.rst
 F:	include/linux/kasan*.h
 F:	lib/test_kasan.c
 F:	mm/kasan/