docs/devel/lockcnt: Convert to rST format

Convert docs/devel/lockcnt.txt to rST format.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20240816132212.3602106-4-peter.maydell@linaro.org
diff --git a/MAINTAINERS b/MAINTAINERS
index 22cc98b..c622433 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -3058,7 +3058,7 @@
 Read, Copy, Update (RCU)
 M: Paolo Bonzini <pbonzini@redhat.com>
 S: Maintained
-F: docs/devel/lockcnt.txt
+F: docs/devel/lockcnt.rst
 F: docs/devel/rcu.txt
 F: include/qemu/rcu*.h
 F: tests/unit/rcutorture.c