Add GitLab Sans and JetBrains Mono fonts

What does this MR do and why?

Closes #1321 (closed)

Replaces existing fonts with GitLab's new fonts: GitLab Sans (used for regular text) and JetBrains Mono (used for code samples). Includes a few small spacing/sizing adjustments.

Screenshots, screen recordings, or links to review app

Some review app links:

• https://1321-inter-font.docs.gitlab-review.app/
• https://1321-inter-font.docs.gitlab-review.app/ee/user/project/repository/
• https://1321-inter-font.docs.gitlab-review.app/ee/api/access_requests.html (lots of code examples here)

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/gitlab-docs/-/blob/main/doc/setup.md.
2. Run yarn install to make sure you have the new fonts.
3. Browse around the site in different browsers (supported browsers). Verify text is readable, spaced appropriately, and sized appropriately.

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.