PostCard description should probably render markdown

[tsheinen]

Hi! I think that the PostCard component description should render the description as markdown. This is identical in the default case and comes with a few nice properties like linking and paragraphs in descriptions.

I did a mock-up on my own copy with the https://www.npmjs.com/package/@astropub/md library.

---
import { Markdown } from '@astropub/md'
---

<div class="transition text-75 mb-3.5 prose-base">
    <Markdown of={description} />
</div>

Something like this pretty much just works. Notably, I did add prose-base as a class because the default p margin is 0 and I thought it looked ugly. I'll not make a PR because this is trivial -- I'm very much not a web dev and will leave the styling to you :)

[saicaca]

Thank you for your suggestion, I will consider it.