Bind ZX -> Save then kill current buffer

Since bury-buffer is already on SPC b z, and isn't a common operation in
day to day Emacs use.

Other useful keybinds:
  ZZ      -> Save then kill current window
  zx      -> kill current buffer (prompts if unsaved)
  SPC b z -> bury buffer
  zn      -> (operator) narrow buffer
  zN      -> widen narrowed buffer

core/autoload/buffers.el

@@ -264,6 +264,13 @@ See `doom-real-buffer-p' for an explanation for real buffers."
 ;;
 ;; Interactive commands
 
+;;;###autoload
+(defun doom/save-and-kill-buffer ()
+  "Save the current buffer to file, then kill it."
+  (interactive)
+  (save-buffer)
+  (kill-current-buffer))
+
 ;;;###autoload
 (defun doom/kill-this-buffer-in-all-windows (buffer &optional dont-save)
   "Kill BUFFER globally and ensure all windows previously showing this buffer