Input: Add missing MISC2-MISC6 gamepad button constants

The MISC2-MISC6 button strings are recognized by SDL3 but were not
mapped in Godot's JoyButton enum, causing 'Unrecognized output string'
warnings for controllers that use these buttons.

This adds:
- JoyButton::MISC2-MISC6 enum values (21-25)
- 'misc2'-'misc6' string mappings in _joy_buttons array
- GDScript bindings for JOY_BUTTON_MISC2 through JOY_BUTTON_MISC6
- Documentation entries in @GlobalScope.xml
- Updates JoyButton::SDL_MAX from 21 to 26

core/input/input.cpp

@@ -66,6 +66,11 @@ static const char *_joy_buttons[(size_t)JoyButton::SDL_MAX] = {
 	"paddle3",
 	"paddle4",
 	"touchpad",
+	"misc2",
+	"misc3",
+	"misc4",
+	"misc5",
+	"misc6",
 };
 
 static const char *_joy_axes[(size_t)JoyAxis::SDL_MAX] = {