graphics/input: Remove getevent README.md and update Kconfig help text.

Remove the in-tree README.md in favor of rst documentation in the
nuttx main repository. Update the Kconfig help text for getevent
to describe what it does, when to use it, and why it is needed.

Signed-off-by: liuhongchao <liuhongchao@xiaomi.com>

Author: HongChao6

graphics/input/Kconfig

@@ -64,7 +64,15 @@ menuconfig GRAPHICS_INPUT_GETEVENT
 	tristate "Enable getevent tool"
 	default n
 	---help---
-		Enable support for a command line getevent tool.
+		The getevent is a command-line input event monitor. It
+		captures and displays real-time events from mouse,
+		touchscreen, and keyboard devices via the input subsystem.
+
+		Use getevent to debug and verify input drivers, inspect
+		raw event data during device bring-up, or diagnose
+		input issues on a running system. It supports automatic
+		device detection in /dev as well as manual device path
+		specification.
 
 if GRAPHICS_INPUT_GETEVENT