Tidy up the markdown under `_coops`

As a result of migrating from the wordpress site to jekyll some of the formatting of the individual co-op "biographies" has been lost. It would be good to go through each of the .md files under _coops and add appropriate markdown markup so that they render in a similar way to the original.

For example _coops/go-free-range.md should render as close as possible to their page on the mirror