fix(NotFound): Improve 404 page styling

[hareetd]

Updates the 404 page to follow the conventions laid out by PatternFly: https://www.patternfly.org/v4/ux-writing/http-404-pages/. I chose not to include the search function or Card header images for simplicity's sake but can try adding those as well if requested.

Fixes #295

Sample:

[andrewazores]

Looks very nice overall. I wonder if it would be worth extracting a function that generates the Card component from an object containing the title, body text, link text, and link path? routes.tsx does essentially this. Something roughly like:

const cards = [
  {
    title: 'About',
    bodyText: 'Access links containing further information about Cryostat',
    linkText: 'View information about Cryostat',
    linkPath: '/about',
  },
];

return (<>

  (
    cards.map(card =>
      <Card className='pf-c-card-not-found'>
        <CardTitle>{card.title}</CardTitle>
        <CardBody>{card.bodyText}</CardBody>
        <CardFooter className='pf-c-card-not-found__footer'>
          <Link to={card.linkPath}>{card.linkText}</Link>
        </CardFooter>
      </Card>
    )
  )

</>);

This would be wrapped inside the rest of the EmptyState component stuff that is already there.

Alternatively, instead of just a generating function like this, you could extract out the card to its own component definition for even cleaner reuse.

[hareetd]

Sounds good, I'll look into the different extraction options.

[hareetd]

With updated text: