On the page https://docs.digitalocean.com/products/app-platform/how-to/add-deploy-do-button/ there is a link with the text "the official page for the DigitalOcean Referral Program." that 404's. It looks like the correct link is now https://www.digitalocean.com/referral-program or a redirect for https://www.digitalocean.com/products/accounts/referrals/ is needed.