docs: require explicit docker image versions

[DeepDiver1975]

Every documented docker image reference on this branch now carries an explicit
version tag, and latest is no longer presented as a valid option. Same change
as #1576 (10.16) and #1575 (master).

owncloud/server:latest does not track the release line documented here — it
follows whatever the newest published image happens to be — so a reader copying
the examples, or a container restart, can end up on a different version than the
documentation describes. There are also no rolling major or minor tags
(owncloud/server:10.15 and :10 both 404 on Docker Hub), so a full version is
the only correct thing to name.

Changes

modules/admin_manual/pages/installation/docker/index.adoc

• Adds a docker-image-version page attribute set to the literal 10.15.3, the
  top of this release line. This intentionally does not reuse
  {latest-server-version}: that is a global site attribute tracking the latest
  release of the product, and the value that wins is the aggregator's
  (owncloud/docs global-attributes.yml), not this branch's antora.yml — which
  is why the published page currently renders OWNCLOUD_VERSION=10.16 here.
• Tags the quick-evaluation docker run (owncloud/server:10.15.3); it was
  untagged, which resolves to latest.
• OWNCLOUD_VERSION in the .env example now names the explicit version, and the
  required-settings table no longer gives latest as its example value.
• Adds a note explaining why OWNCLOUD_VERSION must be pinned.

modules/developer_manual/pages/testing/ui-testing.adoc

• Pins the two untagged selenium/standalone-firefox* pulls, the untagged
  selenium/standalone-chrome-debug run, and both inbucket/inbucket
  references. The selenium pins use 3.141.59-oxygen, matching the chrome pins
  the page already carries because of the issue it links (webUI tests do not run with latest docker-selenium 3.141.59-palladium core#35444);
  inbucket uses 3.1.1, its newest release tag. All five tags verified present
  on Docker Hub.

Testing

npm run antora-local (Node 22, as pinned by this branch's ci.yml) builds with
no errors and no warnings. The rendered page shows OWNCLOUD_VERSION=10.15.3,
owncloud/server:10.15.3, and no occurrence of latest in the settings table;
all five image pins render on the UI testing page.

Note on timing

10.15 is about to be dropped from the published site and archived as
x_archived_10.15, so this content will not be published for long. It is worth
merging anyway: it keeps the branch correct for the window before the
owncloud/docs change lands, and should the branch ever be un-archived it is not
left documenting an image tag that points somewhere else. This PR needs to merge
before the branch is renamed.

[DeepDiver1975]

Added a second commit that fixes useful_pages.adoc, matching #1576.

That page deliberately xrefs into the latest server version rather than this branch, but used this branch's page paths. 11.0 is a docker-only release and reorganized accordingly, so three of the five links break once latest-server-version advances: installation/index.adoc no longer exists, the docker page moved to installation/installing_with_docker.adoc, and maintenance/manual_upgrade.adoc moved under maintenance/upgrading/.

Unlike on 10.16 this cannot fail the site build, since this branch is being dropped from the published site — but it is the same belt-and-braces reasoning as the :latest fix: keep the branch content correct in case it is ever un-archived.

npm run antora-local on this branch: 0 errors, 0 warnings.