Better metadata template listings / previews

[anngvu]

Several user stories here:

1. Overall listing: During a data model design webinar, a contributor was asking if we had metadata template for X, i.e. because the data model design involves importing templates / customizing templates as needed. A clearer listing would help clarify this.

2. Preview for each template: Currently we embed spreadsheets for previews of the metadata templates. Unfortunately, these load more slowly than expected (for example, Imaging Level 2 takes five seconds to display). Moreover, hovering over column headers don't provide descriptions the way a standalone sheets in the browser does. So embedding the spreadsheets was the easiest preview method, and what we went with because there were many other parts of the docs that needed to be prioritized first, but as previews go these get maybe 2 stars out of 5 on helpfulness/informativeness. We should revisit this to try for something that's at least 4 stars on helpfulness.