Better promote edit prediction when signed out (#43665)

Introducing this little popover here that's aimed at better
communicating what Zed's built-in edit prediction feature is and how
much people can get of it for free by purely just signing in.

<img width="600" height="1914" alt="Screenshot 2025-11-27 at 9  50@2x"
src="https://github.com/user-attachments/assets/a7013292-f662-4cae-9a6f-0e69a4a4fa1d"
/>

Release Notes:

- N/A

crates/client/src/zed_urls.rs

@@ -59,3 +59,11 @@ pub fn agent_server_docs(cx: &App) -> String {
         server_url = server_url(cx)
     )
 }
+
+/// Returns the URL to Zed's edit prediction documentation.
+pub fn edit_prediction_docs(cx: &App) -> String {
+    format!(
+        "{server_url}/docs/ai/edit-prediction",
+        server_url = server_url(cx)
+    )
+}