fix(docs): handle missing autoloads file in core doctor

Instead of a long winded backtrace about a missing autoloads file, let
the user know a 'doom sync' is needed.

lisp/cli/doctor.el

@@ -162,6 +162,8 @@ in."
              kill-emacs-query-functions
              kill-emacs-hook)
          (defvar doom-reloading-p nil)
+         (unless (file-exists-p doom-autoloads-file)
+           (user-error "Autoloads file not generated. Did you remember to run 'doom sync'?"))
          (require 'doom-start)
          (doom-initialize-packages))