Document the badge API

I'm not sure we want to make it a fully public API (as opposed to just generating the URLs from the webapp), but if we do, we should mention it on https://docs.softwareheritage.org/devel/swh-web/ and/or https://archive.softwareheritage.org/api/1/

Migrated from T3758 (view on Phabricator)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information