refactor: mvoe config examples to 'rel/config'

mix.exs

@@ -538,7 +538,7 @@ defmodule EMQXUmbrella.MixProject do
     profile = System.get_env("MIX_ENV")
 
     File.cp_r!(
-      "examples",
+      "rel/config/examples",
       Path.join(etc, "examples"),
       force: overwrite?
     )

rebar.config.erl

@@ -525,7 +525,7 @@ etc_overlay(ReleaseType, _Edition) ->
     [
         {mkdir, "etc/"},
         {copy, "{{base_dir}}/lib/emqx/etc/certs", "etc/"},
-        {copy, "examples", "etc/"}
+        {copy, "rel/config/examples", "etc/"}
     ] ++
         lists:map(
             fun