Add styles for code samples

What does this MR do and why?

Styles the last few things for the basic Docs theme:

• Code samples (we're using the Pygments theme, see !17 (comment 1716307835))
  • note: "copy to clipboard" functionality comes later: gitlab-org/gitlab-docs#1764 (moved)
• Touch-ups around the header navbar (just hiding the buttons in the top-right at small-width for now; proper collapsible mobile version comes in #27 (closed))

Closes #4 (closed)

Screenshots, screen recordings, or links to review app

before	after

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. make view
3. Visit a page with code samples (e.g, http://localhost:1313/update/upgrading_from_ce_to_ee/). It should use the Pygments theme shown here, but with a gray background.

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.