docs: add C# code snippet to extensibility.md (#27328)

Fixes #27160.

docs/src/extensibility.md

@@ -156,3 +156,29 @@ page.locator("tag=div").get_by_text("click me").click()
 # we can use it in any methods supporting selectors.
 button_count = page.locator("tag=button").count()
 ```
+
+```csharp
+// Register the engine. Selectors will be prefixed with "tag=".
+// The script is evaluated in the page context.
+await playwright.Selectors.Register("tag", new() {
+  Script = @"
+  // Must evaluate to a selector engine instance.
+  {
+    // Returns the first element matching given selector in the root's subtree.
+    query(root, selector) {
+      return root.querySelector(selector);
+    },
+
+    // Returns all elements matching given selector in the root's subtree.
+    queryAll(root, selector) {
+      return Array.from(root.querySelectorAll(selector));
+    }
+  }"
+});
+
+// Now we can use "tag=" selectors.
+await page.Locator("tag=button").ClickAsync();
+
+// We can combine it with built-in locators.
+await page.Locator("tag=div").GetByText("Click me").ClickAsync();
+```