Update 404 page copy and styles

What does this MR do and why?

This is the first of a few MRs to update the 404 page (next up is #1293 (closed)). This MR addresses a few visual issues with the page:

• Updates wording and adds a link to the archives page (see #849 (comment 1016868532))
• Smaller illustration (as requested in the comment linked above)
• Removes (most) custom styles for the 404 page in favor of GitLab UI utility classes

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/gitlab-docs/-/blob/main/doc/setup.md.
2. Visit the 404: http://localhost:3000/404.html

Note that the review app does not include the Algolia search box (nor will older versions): https://sarahg-404-fixes.docs.gitlab-review.app/404.html. You can see the Algolia search box on a local instance of the site, though.

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.