docs: add EnvironmentService section to Environment doc

docs/en/UI/Angular/Environment.md

@@ -101,3 +101,81 @@ export interface RemoteEnv {
   * `customMergeFn`: You can also provide your own merge function as shown in the example. It will take two parameters, `localEnv: Partial<Config.Environment>` and `remoteEnv` and it needs to return a `Config.Environment` object.
 * `method`: HTTP method to be used when retrieving environment config. Default: `GET`
 * `headers`: If extra headers are needed for the request, it can be set through this field.
+
+## EnvironmentService
+
+` EnvironmentService` is a singleton service, i.e. provided in root level of your application, and keeps the environment in the internal store.
+
+
+### Before Use
+
+In order to use the `EnvironmentService` you must inject it in your class as a dependency.
+
+```js
+import { EnvironmentService } from '@abp/ng.core';
+
+@Component({
+  /* class metadata here */
+})
+class DemoComponent {
+  constructor(private environment: EnvironmentService) {}
+}
+```
+
+You do not have to provide the `EnvironmentService` at module or component/directive level, because it is already **provided in root**.
+
+
+### Get Methods
+
+`EnvironmentService` has numerous get methods which allow you to get a specific value or all environment object.
+
+Get methods with "$" at the end of the method name (e.g. `getEnvironment$`) return an RxJs stream. The streams are triggered when set or patched the state.
+
+#### How to Get Environment Object
+
+You can use the `getEnvironment` or `getEnvironment$` method of `EnvironmentService` to get all of the environment object. It is used as follows:
+
+```js
+// this.environment is instance of EnvironmentService
+
+const environment = this.environment.getAll();
+
+// or
+this.environment.getAll$().subscribe(environment => {
+   // use environment here
+})
+```
+
+#### How to Get API URL
+
+The `getApiUrl` or `getApiUrl$` method is used to get a specific API URL from the environment object. This is how you can use it:
+
+```js
+// this.environment is instance of EnvironmentService
+
+const apiUrl = this.environment.getApiUrl();
+// environment.apis.default.url
+
+this.environment.getApiUrl$("search").subscribe(searchUrl => {
+// environment.apis.search.url
+})
+```
+
+This method returns the `url` of a specific API based on the key given as its only parameter. If there is no key, `'default'` is used.
+
+
+#### How to Set the Environment
+
+`EnvironmentService` has a method named `setState` which allow you to set the state value.
+
+```js
+// this.environment is instance of EnvironmentService
+
+this.environment.setState(newEnvironmentObject);
+```
+
+Note that **you do not have to call this method at application initiation**, because the environment variables are already being stored at start.
+
+#### Environment Properties
+
+Please refer to `Environment` type for all the properties. It can be found in the [config.ts file](https://github.com/abpframework/abp/blob/dev/npm/ng-packs/packages/core/src/lib/models/config.ts#L13).