docs: move locking-specific documents to locking/

Several files under Documentation/*.txt describe some type of
locking API. Move them to locking/ subdir and add to the
locking/index.rst index file.

Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Link: https://lore.kernel.org/r/dd833a[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/locking/index.rst

···
     rt-mutex
     spinlocks
     ww-mutex-design
+    preempt-locking
+    pi-futex
+    futex-requeue-pi
+    hwspinlock
+    percpu-rw-semaphore
+    robust-futexes
+    robust-futex-ABI
 
 .. only::  subproject and html
 

Documentation/locking/rt-mutex.rst

···
 
 RT-mutexes with priority inheritance are used to support PI-futexes,
 which enable pthread_mutex_t priority inheritance attributes
-(PTHREAD_PRIO_INHERIT). [See Documentation/pi-futex.txt for more details
+(PTHREAD_PRIO_INHERIT). [See Documentation/locking/pi-futex.rst for more details
 about PI-futexes.]
 
 This technology was developed in the -rt tree and streamlined for

MAINTAINERS

···
 L:	[e-mail]
 S:	Maintained
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git locking/core
-F:	Documentation/*futex*
+F:	Documentation/locking/*futex*
 F:	include/asm-generic/futex.h
 F:	include/linux/futex.h
 F:	include/uapi/linux/futex.h
 F:	kernel/futex.c
 F:	tools/perf/bench/futex*
-F:	tools/testing/selftests/futex/
+F:	Documentation/locking/*futex*
 
 GASKET DRIVER FRAMEWORK
 M:	Rob Springer <[e-mail]>
···
 S:	Maintained
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/andersson/remoteproc.git hwspinlock-next
 F:	Documentation/devicetree/bindings/hwlock/
-F:	Documentation/hwspinlock.txt
+F:	Documentation/locking/hwspinlock.rst
 F:	drivers/hwspinlock/
 F:	include/linux/hwspinlock.h