Add document for beat export dashboard (#7696)

* Add document for beat export dashboard

Follow up from #7239

* move to command reference

* address review comments

* review comments applied

* apply review feedback

libbeat/docs/command-reference.asciidoc

@@ -14,7 +14,7 @@
 
 :global-flags: Also see <<global-flags,Global flags>>.
 
-:export-command-short-desc: Exports the configuration or index template to stdout
+:export-command-short-desc: Exports the configuration, index template or a dashboard to stdout
 :help-command-short-desc: Shows help for any command
 :keystore-command-short-desc: Manages the <<keystore,secrets keystore>>
 :modules-command-short-desc: Manages configured modules
@@ -77,7 +77,7 @@ endif::[]
 
 [options="header"]
 |=======================
-|Commands | 
+|Commands |
 |<<export-command,`export`>> |{export-command-short-desc}.
 |<<help-command,`help`>> |{help-command-short-desc}.
 |<<keystore-command,`keystore`>> |{keystore-command-short-desc}.
@@ -96,8 +96,8 @@ Also see <<global-flags,Global flags>>.
 ==== `export` command
 
 {export-command-short-desc}. You can use this
-command to quickly view your configuration or the contents of the index
-template.
+command to quickly view your configuration, see the contents of the index
+template or export a dashboard from Kibana.
 
 *SYNOPSIS*
 
@@ -113,6 +113,30 @@ template.
 Exports the current configuration to stdout. If you use the `-c` flag, this
 command exports the configuration that's defined in the specified file.
 
+
+
+*`dashboard`*::
+Exporting a dashboard allows to store a dashboard on disk in a
+module and load it automatically. The following command can be used:
++
+["source","shell",subs="attributes"]
+----
+{beatname_lc} export dashboard --id="dashboard-id" > dashboard.json
+----
++
+The `dashboard-id` can be found in the Kibana URL. By default `export dashboard`
+will write the dashboard to stdout. Above it's written into `dashboard.json` so
+it can later imported again. The file contains the dashboard with all
+visualizations and searches. The index pattern is removed as it is
+expected to be loaded separately for {beatname_uc}.
++
+The generated `dashboard.json` file can be copied into the `kibana/6/dashboard`
+directory of {beatname_lc} and next time +{beatname_lc} setup dashboards+ is
+run the dashboard will be imported.
++
+In case Kibana is not running on `localhost:5061` the {beatname_uc}
+configuration under `setup.kibana` must be adjusted.
+
 [[template-subcommand]]
 *`template`*::
 Exports the index template to stdout. You can specify the `--es.version` and
@@ -176,7 +200,7 @@ Specifies the name of the command to show help for.
 [[keystore-command]]
 ==== `keystore` command
 
-{keystore-command-short-desc}. 
+{keystore-command-short-desc}.
 
 *SYNOPSIS*
 
@@ -690,4 +714,3 @@ the _Beats Platform Reference_ for more information.
 
 *`-v, --v`*::
 Logs INFO-level messages.
-