Updates GuideGroupIndex in PageLayout to use CardContainer to display full width cards

[katydecorah]

This PR updates GuideGroupIndex in PageLayout to use CardContainer to display full width cards. This helps reduce patterns and improve consistency. This PR will also generate a snapshot test for the grouped guides test case.

This PR will also fix an accessibility issue with the NavigationAccordion where the subpages were not properly nested: 6eb9fbf

Fixes #509

How to test

1. nvm use && npm ci && npm start
2. Visit /PageLayout test case, specifically Page layout: group index.
3. npm run start-docs
4. Visit http://localhost:8080/dr-ui/guides/grouped-guides/

QA checklist

• No errors logged to console.
• Accessibility score in Chrome DevTools Audit/Lighthouse is 100% with Lighthouse version: #.#.#.
• Component is accessible at mobile-size viewport.
• Component is accessible at tablet-size viewport.
• Component is accessible at laptop-size viewport.
• Component is accessible at big-monitor-size viewport.

Open the test cases app locally on:

• Chrome, no errors logged to console.
• Firefox, no errors logged to console.
• Safari, no errors logged to console.
• Edge, no errors logged to console.
• Mobile Safari, no errors logged to console.
• Android Chrome, no errors logged to console.

Before merge

• Add entry to CHANGELOG.md to describe changes.
• If updating dependencies or creating a release, run npx browserslist@latest --update-db to update the browser data and commit any changes to package-lock.json.

[colleenmcginnis]

Looks great! Thank you for making our UI more consistent ✨