[site-client] Add markdown rendering for certain fields

[justinfagnani]

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.