Update card width style

What does this MR do and why?

To create a more consistent visual experience (see #331 (closed)), this MR updates card style so that the width of a card is consistent at each breakpoint. On large screens, a card always takes up a third of the available width, no matter how many cards are in the group. On medium screens, a card takes up half the space, and on small screens a card takes up all the space.

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.
2. Check out this branch.
3. Adjust the number of cards in any card group at a variety of screen widths to verify that the width of the card does not vary based on the number of cards.

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.

Closes #331 (closed)