docs: remove root namespace

The root namespace was set to 'cluster' which is not very
accurate since not all the root level configs are 'cluster'
configs

apps/emqx_conf/src/emqx_conf.erl

@@ -196,7 +196,9 @@ schema_module() ->
 -spec gen_doc(file:name_all(), module(), file:name_all(), string()) -> ok.
 gen_doc(File, SchemaModule, I18nFile, Lang) ->
     Version = emqx_release:version(),
-    Title = "# " ++ emqx_release:description() ++ " " ++ Version ++ " Configuration",
+    Title =
+        "# " ++ emqx_release:description() ++ " Configuration\n\n" ++
+            "<!--" ++ Version ++ "-->",
     BodyFile = filename:join([rel, "emqx_conf.template.en.md"]),
     {ok, Body} = file:read_file(BodyFile),
     Opts = #{title => Title, body => Body, desc_file => I18nFile, lang => Lang},

apps/emqx_conf/src/emqx_conf_schema.erl

@@ -64,7 +64,8 @@
     emqx_slow_subs_schema
 ]).
 
-namespace() -> cluster.
+%% root config should not have a namespace
+namespace() -> undefined.
 
 roots() ->
     PtKey = ?EMQX_AUTHENTICATION_SCHEMA_MODULE_PT_KEY,