build: copy example config to etc dir

Makefile

@@ -247,3 +247,4 @@ $(foreach tt,$(ALL_ELIXIR_TGZS),$(eval $(call gen-elixir-tgz-target,$(tt))))
 .PHONY: fmt
 fmt: $(REBAR)
 	@./scripts/erlfmt -w '{apps,lib-ee}/*/{src,include,test}/**/*.{erl,hrl,app.src}'
+	@./scripts/erlfmt -w 'rebar.config.erl'

rebar.config.erl

@@ -393,7 +393,9 @@ etc_overlay(ReleaseType, Edition) ->
     Templates = emqx_etc_overlay(ReleaseType, Edition),
     [
         {mkdir, "etc/"},
-        {copy, "{{base_dir}}/lib/emqx/etc/certs", "etc/"}
+        {copy, "{{base_dir}}/lib/emqx/etc/certs", "etc/"},
+        {copy, "{{base_dir}}/lib/emqx_dashboard/etc/emqx-en.conf.example", "etc/"},
+        {copy, "{{base_dir}}/lib/emqx_dashboard/etc/emqx-zh.conf.example", "etc/"}
     ] ++
         lists:map(
             fun