Add font declaration step for diagrams

What does this MR do and why?

Adds a line to https://docs.gitlab.com/ee/development/documentation/styleguide/#create-a-diagram to provide an optional font-declaration step. Also adds an example to show the ordering.

Related to Track Mermaid graph improvements (#461444 - closed) where I'm gardening this improvement into docs for devopscreate.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.