Add external applications section to custom theme

[rajanarahul93]

Description

This PR adds a basic UI section for showcasing external applications in the GeoNode MapStore client.

• Modified custom_theme.html under geonode/apps/customizations/templates/geonode-mapstore-client/snippets/
• Injected an "External Applications" section with placeholder links pointing to /apps/app1/, /apps/app2/
• Styled minimally for visual consistency

This addresses issue #15 by integrating a visible UI that hints at external application availability and interaction.

Notes

• Links are currently static and for demonstration. They can later be dynamically fetched based on available apps.
• No breaking changes; fallback behavior remains unchanged if external apps are absent.

---

Closes #15