Set up a custom Hugo theme

Set up a new Hugo theme that includes GitLab UI CSS.

• Update FE tooling to copy GitLab UI assets into the theme
• Create the responsive 3-col layout for default pages (note that global nav and ToC are separate issues; just create placeholders for these)
• Add styling for typography, and the global header/footer
• Add anchor links for headers
• Add Help & Feedback footer
• Style global elements that are not custom components (e.g, style code samples, but not tabs). Custom components have individual issues for them later on in the project.
• Styling for codeblocks
• Fix header styling for small screens