[docs-scanner] Ubuntu installation page missing GNOME AppIndicator requirement

[docker-agent]

Files:

• content/manuals/desktop/setup/install/linux/ubuntu.md
• content/manuals/desktop/setup/install/linux/debian.md
• content/manuals/desktop/setup/install/linux/fedora.md
• content/manuals/desktop/setup/install/linux/rhel.md

Issue

The Ubuntu installation page does not explicitly mention the GNOME AppIndicator extension requirement, while the Debian, Fedora, and RHEL pages all explicitly call it out in their Prerequisites sections.

From debian.md (line 28):

For a Gnome Desktop environment, you must also install AppIndicator and KStatusNotifierItem Gnome extensions.

From fedora.md (line 28):

For a GNOME desktop environment you must install AppIndicator and KStatusNotifierItem GNOME extensions.

From rhel.md (line 48):

For a GNOME desktop environment you must install AppIndicator and KStatusNotifierItem GNOME extensions.

From ubuntu.md: No mention of AppIndicator in the Prerequisites section.

Why this matters

A reader installing Docker Desktop on Ubuntu with GNOME will not see the explicit AppIndicator requirement that users of other distributions see. While the general system requirements in _index.md mention this, the inconsistency creates confusion:

1. Ubuntu users might assume they don't need AppIndicator since it's not explicitly listed (unlike other distros)
2. The inconsistency suggests either Ubuntu is different, or the documentation is incomplete
3. Users following the Ubuntu-specific page might miss this requirement entirely if they don't read the general requirements carefully

Suggested fix

Either:

1. Add the explicit AppIndicator requirement to ubuntu.md Prerequisites section (matching Debian/Fedora/RHEL), or
2. Remove the explicit callout from Debian/Fedora/RHEL pages and rely only on the general requirements, or
3. Add a note explaining why Ubuntu is different (if it actually is)

Consistency across distribution pages helps readers understand requirements clearly.

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Found by nightly documentation quality scanner