Use markdown for the entries (at least the description)

[jeroenheijmans]

The data currently sits inside JSON files, which are hard and fragile to author. Instead, at least the description we'd like to have in Markdown. Which would also give us some formatting for longer content.

This ties in with potentially doing #1 at some point, and creating a back-end for the whole thing. But it's not tightly coupled, and we have some options:

• Double-down on Consider making Tierdom reusable by others #1 and build a back-end to support markdown for one or more fields
• Keeping metadata in json and putting only certain fields in markdown files in the repository
• Do everything in markdown files in the repository, and have conventions to pull out all the data at build time

The end goal: making it much easier to add or change content.

Which poses a related issue: how to handle thumbnails and other images? Conventions? Data URL's? External hosting?