Add first draft of sales channel helper page

docs/helpers/sales-channel.md

@@ -0,0 +1,61 @@
+# Sales Channel Helpers
+
+This helper provides utility methods to work with sales channels.
+
+## getStorefrontSalesChannel
+
+The `getStorefrontSalesChannel` method returns the first sales channel of type `Storefront` or null if it does not exists.
+
+```php
+<?php
+
+class MyFixture extends Fixture {
+    public function load(): void {
+        $salesChannel = $this->helper->SalesChannel()->getStorefrontSalesChannel(); // [!code focus]
+    }
+}
+```
+
+## getHeadlessSalesChannel
+
+The `getHeadlessSalesChannel` method returns the first sales channel of type `Headless` or null if it does not exists.
+
+```php
+<?php
+
+class MyFixture extends Fixture {
+    public function load(): void {
+        $salesChannel = $this->helper->SalesChannel()->getHeadlessSalesChannel(); // [!code focus]
+    }
+}
+```
+
+## getProductComparisonSalesChannel
+
+The `getProductComparisonSalesChannel` method returns the first sales channel of type `Product Comparison` (in admin it is called Product Feed) or null if it does not exists.
+
+```php
+<?php
+
+class MyFixture extends Fixture {
+    public function load(): void {
+        $salesChannel = $this->helper->SalesChannel()->getProductComparisonSalesChannel(); // [!code focus]
+    }
+}
+```
+
+## getSalesChannelByType
+
+The `getSalesChannelByType` method takes a type parameter and returns the first sales channel of that specific type or null if it does not exists.
+
+```php
+<?php
+
+class MyFixture extends Fixture {
+    public function load(): void {
+        $salesChannel = $this->helper->SalesChannel()->getSalesChannelByType( // [!code focus]
+            Defaults::SALES_CHANNEL_TYPE_PRODUCT_COMPARISON // [!code focus]
+        ); // [!code focus]
+    }
+}
+```