Add Artifact Registry docs section landing page

What does this MR do?

Creates doc/user/artifact_registry/ with the top-level page for the Artifact Registry beta. This is the first page in a new docs section; the remaining pages land in follow-up merge requests, each adding its own link back to this page.

The page covers the Overview row from the issue's deliverables table:

• What Artifact Registry is, and that it lives at the organization level.
• Beta status, with the "not for production workloads, keep your current registry as the system of record" framing.
• How to request access.
• The three repository types: hosted, remote, and virtual.
• What the beta does not include: no deploy tokens, API-only role assignment, private-only visibility, closed-by-default access, permanent delete, and no organization dashboard.

Related issues

Related to #626825 Parent epic: &21052

Notes for the technical writer

This page sets the pattern for roughly 13 more, so three things are worth settling here rather than 13 times later.

1. group: Container Registry. This matches group::container registry on ee/config/feature_flags/wip/artifact_registry_ui.yml, and that group already has a CodeOwnerRule. Happy to add an Artifact Registry rule and regenerate instead.
2. Navigation. A companion merge request in https://gitlab.com/gitlab-org/technical-writing/docs-gitlab-com adds this under Deploy and release your application > Packages & Registries, as a sibling of Package registry, Container registry, and Virtual registry. Note the URL is user/artifact_registry/, not nested under user/packages/.
3. A possible name collision in the nav. https://gitlab.com/gitlab-org/ops/artifact-registry is scaffolded as a docs source project, but the Hugo mount is commented out in docs-gitlab-com at config/_default/hugo.yaml, and its docs/user/_index.md is an eight-line placeholder. We are not using it for 19.4. Flagging so we do not end up with two nav entries both called Artifact Registry.

There is no ## Related topics section yet on purpose. Forward links to pages that do not exist fail the offline lychee check, so each follow-up merge request adds its own link here as it lands.

Testing

Run against the same tool versions CI uses (markdownlint-cli2 0.22.1, Vale 3.14.2, lychee 0.24.2):

scripts/lint-doc.sh doc/user/artifact_registry/_index.md

• scripts/lint-doc.sh: passed.
• Vale at --minAlertLevel error (the CI gate): 0 errors.
• Vale at --minAlertLevel warning: 0 warnings.
• markdownlint: 0 errors.
• lychee --offline --include-fragments across all of doc/: 0 errors.

Please check the review app for how the {{< details >}} block and the Beta status render.

Author's checklist

• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding a new page, add the product availability details under the H1 topic title.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
• Review by assigned maintainer, who can always request/require the reviews above.