Add shortcode to list the maintained versions

What does this MR do and why?

After the discussion in gitlab-com/gl-infra/delivery#2860 (comment 2508839771), it seems we don't have anywhere listed the maintained versions.

This MR creates a shortcode that reads the current versions from https://gitlab.com/gitlab-org/technical-writing/docs-gitlab-com/-/blob/main/content/versions.json and creates an unordered list, ONLY for the latest (unreleased) version.

Complements the /help fallback.

See gitlab-org/gitlab!191742 (comment 2525198371) for a detailed discussion where we decided to only show this information on the docs site, and only on the latest (unversioned) docs.

TL;DR:

• The maintained versions are only shown under https://docs.gitlab.com/policy/maintenance/#maintained-versions
• If you visit a versioned page like https://docs.gitlab.com/17.11/policy/maintenance/#maintained-versions we link to the latest docs https://docs.gitlab.com/policy/maintenance/#maintained-versions
• If you visit /help, we link to https://docs.gitlab.com/policy/maintenance/#maintained-versions

How to set up and validate locally

1. Configure a local GitLab Docs environment.

2. Check out this branch.

3. In gitlab-org/gitlab, edit doc/policy/maintenance.md and add the following section:

   ## Maintained versions
   
   The following versions are currently maintained:
   
   {{< maintained-versions >}}

From there one, there are two options. One that only shows the list when on the latest docs (unversioned), and the other in all other cases.

For the latest docs:

1. Run Hugo with the CI_COMMIT_REF_NAME set to main:

   CI_COMMIT_REF_NAME=main make view

2. Visit http://localhost:1313/policy/maintenance/#maintained-versions

   It should render like:

   

For all other cases (fallback):

1. Run Hugo:

   make view

2. Visit http://localhost:1313/policy/maintenance/#maintained-versions

   It should render like:

   

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.