Add support for PlantUML diagrams

What does this MR do and why?

Adds support for PlantUML diagrams.

The implementation here borrows heavily from Google's Docsy theme:

• https://github.com/google/docsy/blob/30249c7e8cba6db2f998ef712420e13265b07d03/assets/js/plantuml.js
• https://github.com/google/docsy/blob/30249c7e8cba6db2f998ef712420e13265b07d03/static/js/deflate.js

Closes #14 (closed)

Screenshots, screen recordings, or links to review app

Before	After

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. Visit a page with a plantUML diagram, and verify it looks the same as the Nanoc site. Here are a few examples:

• http://localhost:1313/administration/reference_architectures/25k_users/
• http://localhost:1313/administration/postgresql/replication_and_failover/#architecture
• http://localhost:1313/architecture/blueprints/observability_for_self_managed/#architecture

Tip: You can quickly visit the Nanoc version of these pages by using the link to "17.1" on the Versions dropdown.

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.