Use GitLab Mono over JetBrains Mono

What does this MR do and why?

GitLab Mono is a font based on JetBrains Mono, however:

• it only needs two font files rather than four, because it is based on the variable font
• ligatures are already removed
• the backtick bug, e.g. A is fixed

See also: gitlab!123056 (merged)

Screenshots, screen recordings, or links to review app

Before	After
https://docs.gitlab.com/ee/integration/glab/	https://leipert-gitlab-mono.docs.gitlab-review.app/ee/integration/glab/

The font rendering is the same and the following things look correct:

1. Proper italics (e.g. in list of issues, the f looks a little different than f)
2. Proper font weight (e.g. in --stdin)
3. No ligatures (e.g. in -- in --stdin)

Before	After
https://docs.gitlab.com/ee/user/markdown.html#code-spans-and-blocks	https://leipert-gitlab-mono.docs.gitlab-review.app/ee/user/markdown.html#code-spans-and-blocks

1. The triple backticks (```) render subtly different

How to set up and validate locally

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.

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.