doc: provide error handling documentation

We don't really have docs on how fatal errors are induced
or handled. Provide some documentation that covers:

- Assertions (runtime and build)
- Kernel panic and oops conditions
- Stack overflows
- Other exceptions
- Exception handling policy

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>

include/fatal.h

@@ -4,12 +4,22 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+/** @file
+ *  @brief Fatal error functions
+ */
+
 #ifndef ZEPHYR_INCLUDE_FATAL_H
 #define ZEPHYR_INCLUDE_FATAL_H
 
 #include <arch/cpu.h>
 #include <toolchain.h>
 
+/**
+ * @defgroup fatal_apis Fatal error APIs
+ * @ingroup kernel_apis
+ * @{
+ */
+
 enum k_fatal_error_reason {
 	/** Generic CPU exception, not covered by other codes */
 	K_ERR_CPU_EXCEPTION,
@@ -88,4 +98,6 @@ void k_sys_fatal_error_handler(unsigned int reason, const z_arch_esf_t *esf);
  */
 void z_fatal_error(unsigned int reason, const z_arch_esf_t *esf);
 
+/** @} */
+
 #endif /* ZEPHYR_INCLUDE_FATAL_H */