[SIEM] Adds ECS link to help menu (#57104)

## Summary

Adds link to ECS docs in SIEM HelpMenu as requested by @MikePaquette. 🙌 📜 

<img src="https://user-images.githubusercontent.com/2946766/74039051-71ab9a80-497e-11ea-92e0-e69988af8fdc.png" width=300>
 

### Checklist

Delete any items that are not applicable to this PR.

- [x] Any text added follows [EUI's writing guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses sentence case text and includes [i18n support](https://github.com/elastic/kibana/blob/master/packages/kbn-i18n/README.md)
- [ ] ~[Documentation](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#writing-documentation) was added for features that require explanation or tutorials~
- [ ] ~[Unit or functional tests](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#cross-browser-compatibility) were updated or added to match the most common scenarios~
- [ ] ~This was checked for [keyboard-only and screenreader accessibility](https://developer.mozilla.org/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Accessibility#Accessibility_testing_checklist)~
- [ ] ~This renders correctly on smaller devices using a responsive layout. (You can test this [in your browser](https://www.browserstack.com/guide/responsive-testing-on-local-server)~
- [ ] ~This was checked for cross-browser compatibility, [including a check against IE11](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#cross-browser-compatibility)~

### For maintainers

- [ ] ~This was checked for breaking API changes and was [labeled appropriately](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#release-notes-process)~

x-pack/legacy/plugins/siem/public/components/help_menu/index.tsx

@@ -24,10 +24,24 @@ export const HelpMenu = React.memo(() => {
           href: docLinks.links.siem.guide,
           iconType: 'documents',
           linkType: 'custom',
+          target: '_blank',
+          rel: 'noopener',
+        },
+        {
+          content: i18n.translate('xpack.siem.chrome.helpMenu.documentation.ecs', {
+            defaultMessage: 'ECS documentation',
+          }),
+          href: `${docLinks.ELASTIC_WEBSITE_URL}guide/en/ecs/current/index.html`,
+          iconType: 'documents',
+          linkType: 'custom',
+          target: '_blank',
+          rel: 'noopener',
         },
         {
           linkType: 'discuss',
           href: 'https://discuss.elastic.co/c/siem',
+          target: '_blank',
+          rel: 'noopener',
         },
       ],
     });