doxygen: re-org module groups (#10197)

Match the organization of modules (groups) with
that of user guide.

Preliminary arrangement. This patch is just a
framework arrangement. Details need to be continued.

P.S., in this patch, adding numerical prefixes to
the files in documentation/0.doxygen is to meet the
need of displaying sorting in HTML pages.

Signed-off-by: Chen Wang <unicorn_wang@outlook.com>

documentation/0.doxygen/core/systeminit.h

@@ -0,0 +1,19 @@
+/*
+ * This file is only used for doxygen document generation.
+ */
+
+/**
+ * @ingroup group_SystemInit
+ *
+ * This function will initialize user application.
+ *
+ * This function will be invoked when system initialization and system scheduler
+ * has not started. User can allocate memory, create thread, semaphore etc. However,
+ * user shall not suspend 'current' thread.
+ */
+void rt_application_init();
+
+/**
+ * @ingroup group_SystemInit
+ */
+void rt_system_heap_init(void* begin_addr, void* end_addr);