update styles for 404 page

[tjheffner]

Page to edit

url: any 404 page

Origin of request (internal/stakeholder/user feedback)

https://dsva.slack.com/archives/CBU0KDSB1/p1704994672158839

Regression on the global 404 page was discovered. It was referencing one-off styles that were removed from Formation. The markup has been updated to use classes available in the design system.

Description of what's needed (edits/link changes/additions)

The va.gov-content 404.md page that includes this template has also been updated in this PR: department-of-veterans-affairs/vagov-content#902

[randimays]

@tjheffner Do we need to merge the vagov-content PR first in order to see the styles updated on this review instance?

[tjheffner]

@randimays to see the updates for the search form on the 404 yeah, that one would probably need to be merged first. but afaik there's no good way to test vagov-content updates besides checking both branches out locally and doing a local content build 🙃

without merging that pr first, review instances for this branch would only have updates to the "common and popular" portion of the template.

fwiw, the styles + markup between these two PRs are the same as the changes to the 404 page in next-build & this other app's not found page

[randimays]

Nice, thanks @tjheffner!