Descriptions for groups and snapshots

[captainbrosset]

We're thinking about publishing groups and snapshots, but (unless I'm mistaken) our groups and snapshots don't have descriptions yet.
For a consumer to use those groups/snapshots in a product, it would make sense for them to have a short description, just like features do.

At least I know I would love to display groups on https://captainbrosset.github.io/web-features-explorer/ as a way to navigate some of the features.

[ddbeck]

This is likely to blocked by #648 (for groups, at least).