[Keymap] Reorganization, cleanup and readmes for drashna code (#15617)

users/drashna/callbacks.md

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+# Custom Userspace Callback Functions
+
+Specifically QMK works by using customized callback functions for everything. This allows for multiple levels of customization. 
+
+`matrix_scan` calls `matrix_scan_quantum`, which calls `matrix_scan_kb`, which calls `matrix_scan_user`. 
+`process_record` calls a bunch of stuff, but eventually calls `process_record_kb` which calls `process_record_user`
+The same goes for `matrix_init`, `layer_state_set`, `led_set`, and a few other functions.  
+
+All (most) `_user` functions are handled here, in the userspace instead.  To allow keyboard specific configuration, I've created `_keymap` functions that can be called by the keymap.c files instead.
+
+This allows for keyboard specific configuration while maintaining the ability to customize the board.
+
+My [Ergodox EZ Keymap](https://github.com/qmk/qmk_firmware/blob/master/layouts/community/ergodox/drashna/keymap.c) is a good example of this, as it uses the LEDs as modifier indicators.
+
+You can see a list of these files in [callbacks.c](callbacks.c), or a shortend list here 
+
+```c
+__attribute__((weak)) void matrix_init_keymap(void) {}
+void                       matrix_init_user(void) {
+    matrix_init_keymap();
+}
+
+__attribute__((weak)) void keyboard_post_init_keymap(void) {}
+void                       keyboard_post_init_user(void) {
+    keyboard_post_init_keymap();
+}
+
+__attribute__((weak)) void matrix_scan_keymap(void) {}
+void                       matrix_scan_user(void) {
+    matrix_scan_keymap();
+}
+
+__attribute__ ((weak)) bool process_record_keymap(uint16_t keycode, keyrecord_t *record) { return true; }
+bool                        process_record_user(uint16_t keycode, keyrecord_t *record) {
+    if (!process_record_keymap(keycode, record)) { return false; }
+    return true;
+}
+
+__attribute__((weak)) layer_state_t layer_state_set_keymap(layer_state_t state) { return state; }
+layer_state_t                       layer_state_set_user(layer_state_t state) {
+    state = layer_state_set_keymap(state);
+    return state;
+}
+
+__attribute__ ((weak)) void led_set_keymap(uint8_t usb_led) {}
+void                        led_set_user(uint8_t usb_led) {
+    led_set_keymap(usb_led);
+}
+
+__attribute__ ((weak)) void suspend_power_down_keymap(void) {}
+void                        suspend_power_down_user(void) {
+    suspend_power_down_keymap();
+}
+
+__attribute__ ((weak)) void suspend_wakeup_init_keymap(void) {}
+void                        suspend_wakeup_init_user(void) {
+    suspend_wakeup_init_keymap();
+}
+
+
+__attribute__ ((weak)) void shutdown_keymap(void) {}
+void                        shutdown_user (void) {
+    shutdown_keymap();
+}
+
+__attribute__ ((weak)) void eeconfig_init_keymap(void) {}
+void                   eeconfig_init_user(void) {
+    eeconfig_update_user(0);
+    eeconfig_init_keymap();
+}
+```