fix: correct broken xrefs in useful_pages (#1541)

[DeepDiver1975]

Problem

On useful_pages.html the Installation with Docker link renders as a dead on-page anchor instead of a real link:

https://doc.owncloud.com/server/next/admin_manual/useful_pages.html#server:admin_manual:installation/installing_with_docker.adoc

Fixes #1541

Root cause

The four links in useful_pages.adoc carried a server: component prefix without a version coordinate:

xref:server:admin_manual:installation/installing_with_docker.adoc[Installation with Docker]

Per Antora's resolution rules, a component-qualified xref with no version resolves against the latest non-prerelease version of that component. Since next is flagged prerelease: true in antora.yml, in the multi-version production build (owncloud/docs) these xrefs resolved against the current release rather than next.

installing_with_docker.adoc was newly added/renamed in next (from installation/docker/index.adoc) in #1511, so the target did not exist in the release version. Antora could not resolve it and emitted a self-anchor fallback — exactly the broken URL reported in the issue.

This is why the page builds clean locally: a local build only contains next, so "latest" == next, where the file exists.

Fix

useful_pages.adoc itself lives in the server component, so dropping the server: prefix makes these same-component xrefs resolve against the current page's version (where every target exists). This also matches how all other references to these pages are written across the repo.

-* xref:server:admin_manual:installation/installing_with_docker.adoc[Installation with Docker]
+* xref:admin_manual:installation/installing_with_docker.adoc[Installation with Docker]

Note: the previous {latest-server-version}@server: form (removed in #1511) could not be restored, because latest-server-version is defined unquoted in antora.yml and parses as a YAML object, rendering as [object Object]. See follow-up below.

Verification

Local Antora build confirms all four links now render as proper anchors with no [object Object] and no #server:admin_manual self-anchor fallback:

• installation/installing_with_docker.html ✅ (the reported link)
• configuration/server/occ_command.html ✅
• configuration/server/caching_configuration.html#small-organization-single-server-setup ✅
• maintenance/upgrading/manual_upgrade.html ✅

Follow-up (out of scope)

The broken latest-server-version attribute ([object Object] from unquoted YAML in antora.yml) also leaks into public_link_shares.html and the collabora_secure_view references. That's a separate defect worth its own fix.

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