Document environment_grouped in app show

Add documentation for the application configuration fields returned by the App Show endpoint and CLI. The API reference now documents the App Show response, describing `environment` (user-set env vars) and `environment_grouped` (env vars grouped by origin with `user` and `service` keys). The CLI command docs (epinio app show) were updated to note the `-o json` output includes these fields and how to use `environment_grouped` to distinguish user vs service-provided variables. The configurations guide gained a short note pointing users to use the CLI JSON output or the API `?grouped=true` query to view environment variables by origin.

Author: psanghvi17

docs/references/api.md

@@ -22,3 +22,10 @@ The endpoints are documented in the [OpenAPI Spec](https://www.openapis.org/) fo
 __Make sure__ you change to the Epinio release you have deployed.
 
 There are various tools you can use to easily browse the API (e.g. https://editor.swagger.io/). If you have a Golang environment set up locally, you may find it easier to run `make swagger swagger-serve` from the root of the Epinio repository.
+
+### App Show response (GET application)
+
+The App Show endpoint returns application details including `configuration`. The configuration includes:
+
+- **`environment`** — Environment variables set by the user (key-value map).
+- **`environment_grouped`** — Environment variables grouped by origin (user-set vs service-provided). Has the same structure as `GET .../environment?grouped=true`, with `user` and `service` keys each containing an `EnvVariableMap`. Use this when you need to distinguish user-defined variables from those injected by bound services.

docs/references/commands/cli/app/epinio_app_show.md

@@ -9,7 +9,12 @@ doc-persona: [epinio-developer, epinio-operator]
 ---
 ## epinio app show
 
-Describe the named application
+Describe the named application. Shows details such as status, instances, routes, bound configurations, and environment.
+
+With `-o json`, the response includes application `configuration` with:
+
+- **`environment`** — User-set environment variables (key-value map).
+- **`environment_grouped`** — Environment variables grouped by origin (`user` and `service`), same structure as [epinio app env show](env/epinio_app_env_show.md) when using the API with `?grouped=true`. Use this to tell user-defined variables apart from those injected by bound services.
 
 ```
 epinio app show NAME [flags]

docs/references/configurations.md

@@ -75,3 +75,7 @@ filesystem of that application, namely:
 
 The application can now read these files and use their content to connect to the configuration,
 authenticate, and then use it.
+
+### Viewing environment variables by origin
+
+Environment variables from bound configurations are also exposed as application environment variables. To see user-set variables separately from service-provided ones (from bound configurations), use [epinio app show](commands/cli/app/epinio_app_show.md) with `-o json` and read `configuration.environment_grouped`, or use the API's environment endpoint with `?grouped=true`.