docs: add java/csharp code snippets for FormData (#28951)

Fixes https://github.com/microsoft/playwright/issues/28811

---------

Signed-off-by: Max Schmitt <max@schmitt.mx>
Co-authored-by: Dmitry Gozman <dgozman@gmail.com>

docs/src/api/class-formdata.md

@@ -27,6 +27,34 @@ Creates new instance of [FormData].
 
 Sets a field on the form. File values can be passed either as `Path` or as `FilePayload`.
 
+```java
+import com.microsoft.playwright.options.FormData;
+...
+FormData form = FormData.create()
+    // Only name and value are set.
+    .set("firstName", "John")
+    // Name and value are set, filename and Content-Type are inferred from the file path.
+    .set("profilePicture1", Paths.get("john.jpg"))
+    // Name, value, filename and Content-Type are set.
+    .set("profilePicture2", new FilePayload("john.jpg", "image/jpeg", Files.readAllBytes(Paths.get("john.jpg"))));
+    .set("age", 30);
+page.request().post("http://localhost/submit", RequestOptions.create().setForm(form));
+```
+
+```csharp
+var multipart = Context.APIRequest.CreateFormData();
+// Only name and value are set.
+multipart.Set("firstName", "John");
+// Name, value, filename and Content-Type are set.
+multipart.Set("profilePicture", new FilePayload()
+{
+    Name = "john.jpg",
+    MimeType = "image/jpeg",
+    Buffer = File.ReadAllBytes("john.jpg")
+});
+await Page.APIRequest.PostAsync("https://localhost/submit", new() { Multipart = multipart });
+```
+
 ### param: FormData.set.name
 * since: v1.18
 - `name` <[string]>