Add styles for YouTube embeds and stylized links

What does this MR do and why?

Closes #58 (closed)

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. View some pages with YouTube embeds and links, such as:

• http://localhost:1313/ci/
• http://localhost:1313/development/fe_guide/onboarding_course/lesson_1/
• http://localhost:1313/user/project/issue_board/

Videos should resize appropriately and maintain their 16/9 aspect ratio if you resize your window.

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.