Create <Caption /> component

[jvgaultney]

This would be used for rich text content typically below an image or other content that should be styled to look like a caption. The styling should reasonably match that used for and can be used with it (either before or after). It would support basic markdown styling (bold, italic), even markdown-style links, but not multiple paragraphs or more complex elements (e.g. tables).

Proposed spec:

<Caption text="This is the caption text and _may_ include basic [markdown](https://www.markdownguide.org/) styling." />

Since there is already an NTML element we may need to use a different component name (but maybe not).

The component will need to push the content through astro's markdown processor.

This will also address other problems with captions: #122 #133