docs: Update Firebase authentication setup and configuration documentation

[Zfinix]

Problem

The Sign in with Firebase docs had no explicit migration commands (just "run serverpod generate"), no troubleshooting page, no parameter reference table, and the caution block redirected users to a complex setup page before they could get started.

Solution

• Setup page -- Removed the caution block and inlined the auth module setup link with a note that projects scaffolded with Serverpod 3.4 or later already include it. Renamed "Create your credentials" to "Get your credentials". Added explicit migration commands (serverpod generate + serverpod create-migration + --apply-migrations) with a crash warning. Added YAML indentation warning to the credentials section. Added troubleshooting link at the bottom.
• Configuration page -- Added a FirebaseIdpConfig parameter reference table with passwords.yaml key column and environment variable equivalents section.
• Troubleshooting page (new) -- Setup checklist and 8 documented failure scenarios with cause and fix for each, including missing migration, token verification, sign-out sync, phone-only validation, and FlutterFire configuration issues.