add info on authorization to Angular UI docs

docs/en/UI/Angular/Authorization.md

@@ -0,0 +1,28 @@
+# Authorization in Angular UI
+
+OAuth is preconfigured in Angular application templates. So, when you start a project using the CLI (or Suite, for that matter), authorization already works. You can find **OAuth configuration** in the _environment.ts_ files.
+
+```js
+import { Config } from '@abp/ng.core';
+
+const baseUrl = 'http://localhost:4200';
+
+export const environment = {
+  // other options removed for sake of brevity
+
+  oAuthConfig: {
+    issuer: 'https://localhost:44305',
+    redirectUri: baseUrl,
+    clientId: 'MyProjectName_App',
+    responseType: 'code',
+    scope: 'offline_access MyProjectName',
+  },
+
+  // other options removed for sake of brevity
+} as Config.Environment;
+
+```
+
+This configuration results in an [OAuth authorization code flow with PKCE](https://tools.ietf.org/html/rfc7636) and we are using [angular-oauth2-oidc library](https://github.com/manfredsteyer/angular-oauth2-oidc#logging-in) for managing OAuth in the Angular client.
+
+According to this flow, the user is redirected to an external login page which is built with MVC. So, if you need **to customize the login page**, please follow [this community article](https://community.abp.io/articles/how-to-customize-the-login-page-for-mvc-razor-page-applications-9a40f3cd).