[site-client] Add markdown rendering for certain fields

Many of the text fields in the custom elements manifest nominally support markdown.

Some are naturally full markdown fields, like the readme fields for packages. Others are single-line-ish fields like package description or element summary. I'm not sure if there's a markdown subset for use in single-line contexts (like what LaTeX might call inline?) - we should see if there is if GitHub or npm allow some markdown in similar contexts.