Add specimen pages

Based on https://gitlab.com/gitlab-org/frontend/specimen-builder

This will build four specimen pages:

• https://gitlab-org.gitlab.io/frontend/fonts/specimen/gitlab-sans
• https://gitlab-org.gitlab.io/frontend/fonts/specimen/gitlab-sans-italic
• https://gitlab-org.gitlab.io/frontend/fonts/specimen/gitlab-mono
• https://gitlab-org.gitlab.io/frontend/fonts/specimen/gitlab-mono-italic

For example:

These then can easily be screenshot for regression testing.