More providers

Read docs/product/users.md and the sign-in decision record in docs/decisions. Say which one more provider these users really have, such as LinkedIn, GitLab or Discord, or say none and stop. Read its current docs and report whether it speaks OIDC or OAuth, which stable id to use as the subject, whether it reports a verified address and whether it has to approve the application first. LinkedIn, for one, needs a product request on the application and does not verify identity. Add it the way Google and GitHub were added, on the hosted provider or in your own service, with the same linking rule. Keep the login page to the few buttons these users will recognise, and tell me if this one pushes it past four. Add what you found to the decision record.