Add shortcode for GitLab SVGs

What does this MR do and why?

Adds a shortcode for GitLab SVG icons.

Part 1 of 2 to close #12 (closed)

How to set up and validate locally

⚠ You should clone this project to a separate directory from your regular GitLab projects, as it will overwrite docs content files in the gitlab, gitlab-runner, etc directories at the same level.

1. Check out this branch: git fetch origin 12-gitlab-svgs && git checkout 12-gitlab-svgs
2. Configure a local GitLab Docs Hugo environment and start the preview server: https://gitlab.com/gitlab-org/technical-writing-group/gitlab-docs-hugo/-/blob/main/doc/setup.md.
3. Review the Shortcode reference page, it now includes Icon: http://localhost:1313/shortcodes/.
4. Edit or add a regular page, and try creating a few icons or modifying existing ones.

For example, you could edit this page locally and make this change:

Old format:

- **Open live environment** (**{external-link}**) - Opens the URL of the application deployed in production
- **Monitoring** (**{chart}**) - Opens the metrics page where Prometheus collects data

New format:

- **Open live environment** ({{< icon name="external-link" >}}) - Opens the URL of the application deployed in production
- **Monitoring** ({{< icon name="chart" >}}) - Opens the metrics page where Prometheus collects data

View the page locally: http://localhost:1313/topics/autodevops/cloud_deployments/auto_devops_with_eks/

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.