Update new behavior example code

[joelspadin]

Some of the example code at https://zmk.dev/docs/development/new-behavior#creating-the-driver-c is either outdated or incomplete. The following issues should be fixed:

1. #if DT_HAS_COMPAT_STATUS_OKAY(DT_DRV_COMPAT) in the C file should be replaced with

   config ZMK_BEHAVIOR_<NAME>
       bool
       default y
       depends on DT_HAS_ZMK_BEHAVIOR_<NAME>_ENABLED
   

   in Kconfig.behaviors and

   target_sources_ifdef(CONFIG_ZMK_BEHAVIOR_<NAME> app PRIVATE src/behaviors/<file>.c)

   in CMakeLists.txt
2. The code example with BEHAVIOR_DT_INST_DEFINE(0, ... shows how to create a behavior that only supports one instance. It includes a comment that implies that simply switching 0 to n will add support for multiple instances, but this is incorrect. The correct structure for such a behavior is

   #define MY_BEHAVIOR_INST(n) \
       static const struct my_behavior_config my_behavior_config_##n = { ... }; \
       static struct my_behavior_data my_behavior_data_##n = { ... }; \
       BEHAVIOR_DT_INST_DEFINE(n, my_behavior_init, NULL, &my_behavior_data_##n, &my_behavior_config_##n, \
                               POST_KERNEL, CONFIG_KERNEL_INIT_PRIORITY_DEFAULT, &my_behavior_driver_api);
   
   DT_INST_FOREACH_STATUS_OKAY(MY_BEHAVIOR_INST)

   We could either make this the only example, then describe how to simplify it if your behavior only supports one instance, or we could have examples of both formats (though having two examples could be more confusing and harder to maintain).