gallery: going out of maintenance

[DeepDiver1975]

owncloud/gallery is being archived and will no longer be maintained.
Please update the documentation to remove or deprecate its pages and references.

• Archived repo: https://github.com/owncloud/gallery
• README archive notice: docs: mark repository as archived and unmaintained gallery#880

[DeepDiver1975]

Investigated gallery references across all built content repos/branches (verified against live GitHub, not just the local Antora cache).

Findings

The gallery app was already deprecated in ownCloud Server 10.3 (superseded by the Files Media Viewer app), so existing references were already deprecation notices. All actionable references live in the docs-server content repo (nothing to change in this aggregator repo). Opened a PR to reflect the archival:

owncloud/docs-server#1559

• admin_manual/.../apps_supported.adoc — note Gallery / Files VideoPlayer are deprecated, archived, and unmaintained.
• classic_ui/pages/apps/media_viewer_app.adoc — note the replaced apps are no longer maintained.
• Removed orphaned classic_ui/images/gallery-1.png and gallery-2.png (no page referenced them).

Intentionally left unchanged

• Historical release notes in docs-main (server_release_notes.adoc) documenting the 10.3 Gallery→Media Viewer transition — accurate historical record.
• Marketplace links (marketplace.owncloud.com/apps/gallery) — the ownCloud Marketplace links appear broadly dead now (even the current files_mediaviewer link returns 404), which is a separate pre-existing issue rather than something the gallery archival caused.

Let me know if you'd also like the historical release-note wording adjusted or the marketplace-link rot tracked separately.