Add vulnerabilities use case page

Created by: katjuell

Closes #5172 (closed) by adding a vulnerabilities page under a new use-cases pages directory. Also moves pages/use-cases.tsx to pages/use-cases/index.tsx.

What are the acceptance criteria?

• Page matches design specs
• ^^ on mobile
• All meta is correct
• Carousel behavior everywhere is correct (code-insights, cloud-beta, this page. We use this component on all of these pages, so any changes we make to it need to look okay in all of these places).
• BackButton on get started pages looks okay (that was updated and turned into a shared component).
• Adds link to vulnerabilities page from main index page.

To Test

• Check out the branch and/ or deploy preview and navigate to /use-cases/vulnerabilities.
• Check the above criteria.

Notes

• We are using Material Design icons here, since this is the icon library we use across our properties. The designs use Phosphor, but CPT has determined that we will not be using icons from this library until we are ready to make a site-wide change. Until that point, we will be using a single icon library for the sake of consistency.
• The BlogListItem section behavior on mobile (Related resources section) matches the behavior of that component on code-insights on mobile, which CPT would like to keep consistent between the pages (using flex-column and full-width images on mobile).
• The gradient background on the quote carousel section now matches what's in #5179 — again, for consistency.

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