Add link to supplier A–Z
Add link to supplier A–Z

Since this ([soon won't be](https://github.com/alphagov/digitalmarketplace-buyer-frontend/pull/150)) behind a feature flag we can reliably assume, in this app, that it exists and therefore link to it.
alphagov/digitalmarketplace-supplier-frontend,alphagov/digitalmarketplace-supplier-frontend,alphagov/digitalmarketplace-supplier-frontend,alphagov/digitalmarketplace-supplier-frontend