Support link-modules and add docs link (#368)

* Allow link type modules

* Add docs module

* Allow buttons to render as <a>

* Update button readme

* Use link buttons in module sidebar when needed

src/modules/docs/index.ts

@@ -0,0 +1,8 @@
+import { defineModule } from '@/modules/define';
+
+export default defineModule(({ i18n }) => ({
+	id: 'docs',
+	name: i18n.t('help_and_docs'),
+	icon: 'help',
+	link: 'https://docs.directus.io',
+}));