docs: clarify Ubuntu AppIndicator requirement (#25721)

## Description

Adds the GNOME AppIndicator and KStatusNotifierItem GNOME extension
requirement to the Ubuntu installation guide, making it consistent with
the Debian, Fedora, and RHEL installation pages.

## Related issues or tickets

Fixes #25701

## Reviews

- [ ] Technical review
- [ ] Editorial review
- [ ] Product review

Co-authored-by: flamercrust <flamercrust@gmail.com>

Author: devendrakushwah80

content/manuals/desktop/setup/install/linux/ubuntu.md

@@ -30,6 +30,7 @@ To install Docker Desktop successfully, you must:
 
 - Meet the [general system requirements](_index.md#general-system-requirements).
 - Have an x86-64 system with Ubuntu 26.04 LTS or 24.04 LTS.
+- For a GNOME desktop environment, you must install AppIndicator and KStatusNotifierItem [GNOME extensions](https://extensions.gnome.org/extension/615/appindicator-support/).
 - If you're not using GNOME, you must install `gnome-terminal` to enable terminal access from Docker Desktop:
   ```console
   $ sudo apt install gnome-terminal