Add migration for availability details

What does this MR do and why?

Migrates availability details (tier/status/offering info) to the Hugo shortcode version.

Note that this does not include version history, which is a separate element (!83 (merged)).

Related:

• Issue: Port markdown filter: Availability details (#11 - closed) • Sarah German
• Frontend MR: Add shortcodes for availability and history (!81 - merged) • Sarah German • 17.1

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/technical-writing-group/gitlab-docs-hugo/-/blob/main/doc/setup.md.
2. Checkout this branch
3. Fetch up-to-date content, but don't run any migrations: REMOVE_BEFORE_CLONE=true go run cmd/gldocs/main.go clone
4. Run this migration only: go run cmd/gldocs/main.go migrate alerts ../gitlab/doc
5. View the diff: git -C ../gitlab diff

Expected result: In addition to the existing alert migrations for WARNING, INFO, and DISCLAIMER alerts, we should now see diffs for DETAILS alerts. Diffs should be something like this:

Before:

DETAILS:
**Tier:** Free, Premium, Ultimate
**Offering:** Self-managed

After:

{{< details >}}

- Tier: Free, Premium, Ultimate
- Offering: Self-managed

{{< /details >}}

We can also test the end-to-end build with the migration:

1. Run the full build process: REMOVE_BEFORE_CLONE=true make clone-docs-projects (runs the initial clones + all migrations)
2. Run the preview server: make view
3. Browse around the site and verify that the DETAILS content matches its appearance on the existing site.

Nanoc site	Hugo site

Example pages:

• Nanoc: https://docs.gitlab.com/ee/user/ai_features.html#code-suggestions / Hugo: http://localhost:1313/user/ai_features/#code-suggestions
• Nanoc: https://docs.gitlab.com/ee/administration/auth/ / Hugo: http://localhost:1313/administration/auth/
• Nanoc: https://docs.gitlab.com/ee/user/project/integrations/webhooks.html / Hugo: http://localhost:1313/user/project/integrations/webhooks/

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.