Add spacing around example forms and DLs

[planktonic]

When using a-dl or .o-table, we almost always want space above and below (xl). The examples should reflect this.

Also, DLs could do with some sectioning, or reduced borders, or something of a grid layout — something to make the layout clearer when there’s e.g. only a couple of entries. Right now the info feels too much like it’s floating in space.