Problem

The provider docs documented the pre-3.5 migration flow (serverpod generate + serverpod create-migration + dart run bin/main.dart --apply-migrations). On 4 the workflow is serverpod start, which generates the code and runs the server. Migrations are then created and applied from the TUI: press M to create one and A to apply it.

Solution

Switch the auth provider docs to the serverpod start + M/A flow, consistently across:

• The general identity-providers setup section.
• All provider setup pages: Google, Apple, GitHub, Firebase, Email, Anonymous, Facebook, Microsoft, Passkey.
• The provider troubleshooting pages (setup steps, checklists, and Cause/Resolution blocks).

The explicit commands stay documented on the migrations reference and the --role maintenance / --apply-repair-migration callsites.

Part of #565