docs: update environment docs (#682)

Author: scarletteliza

docs/docs/basics/enviroments.md

@@ -0,0 +1,57 @@
+---
+sidebar_position: 7
+title: 🌱 Environments
+---
+
+# Environments 🌱
+
+---
+
+There are many ways that environments can be configured in a Dart Frog application. The
+easiest way is to use environment variables via the Dart SDK.
+
+As seen in the [dependency injection docs](dependency-injection.md), middleware can be used to provide dependencies to an application.
+
+Following this approach, the snippet shows how a database client can be configured with different environments.
+
+```dart
+Handler middleware(Handler handler) {
+  return handler
+      .use(provider<CardsRepository>((_) {
+        return DatabaseClient(
+          dbUrl: Platform.environment['DB_URL'],
+          dbUser: Platform.environment['DB_USER'],
+          dbPassword: Platform.environment['DB_PASSWORD'],
+        );
+      }),
+    );
+}
+```
+
+When running the server, these environment variables can be passed along directly to Dart Frog commands:
+
+Development server:
+
+```bash
+DB_URL=... DB_USER=... DB_PASSWORD=... dart_frog server
+```
+
+Production server:
+
+```bash
+DB_URL=... DB_USER=... DB_PASSWORD=... dart build/server.dart
+```
+
+These variables can also be exported in the current session:
+
+```bash
+EXPORT DB_URL=...
+EXPORT DB_USER=...
+EXPORT DB_PASSWORD=...
+```
+
+:::warning
+Accessing variables through `String.fromEnvironment` in a Dart Frog application will not work.`String.fromEnvironment` is meant to accesses variables set by the Dart
+compiler or runtime, which does not apply to a Dart Frog application. Instead, use
+`Platform.environment`.
+:::