docs(java): add BrowserContext.backgroundPage(s)

Refereence microsoft/playwright-java#1530

docs/src/api/class-browsercontext.md

@@ -64,7 +64,7 @@ await context.CloseAsync();
 
 ## event: BrowserContext.backgroundPage
 * since: v1.11
-* langs: js, python
+* langs: js, python, java
 - argument: <[Page]>
 
 :::note
@@ -73,6 +73,13 @@ Only works with Chromium browser's persistent context.
 
 Emitted when new background page is created in the context.
 
+```java
+Page backgroundPage = context.waitForBackgroundPage(() -> {
+  page.getByText("activate extension").click();
+});
+System.out.println(backgroundPage.evaluate("location.href"));
+```
+
 ```js
 const backgroundPage = await context.waitForEvent('backgroundpage');
 ```
@@ -441,7 +448,7 @@ Script to be evaluated in all pages in the browser context. Optional.
 
 ## method: BrowserContext.backgroundPages
 * since: v1.11
-* langs: js, python
+* langs: js, python, java
 - returns: <[Array]<[Page]>>
 
 :::note