docs: Update Google Sign-In setup and configuration documentation

[Zfinix]

Problem

The Sign in with Google docs lacked screenshots for the Google Cloud Console setup steps, had no clear separation between what iOS, Android, and Web each require, and didn't explain key gotchas (YAML indentation, SHA-1 debug vs release mismatch, missing People API). There was also no troubleshooting page and the migration steps were incomplete.

Solution

• Setup page -- Added 4 screenshots for the People API, Audience, iOS OAuth client, and Android OAuth client creation steps. Added a platform requirements table, step-by-step numbered instructions for each platform, explicit migration commands (serverpod create-migration), Firebase disambiguation note, and warnings for common mistakes.
• Configuration page -- Added a GoogleIdpConfig parameter reference table with passwords.yaml key column and environment variable equivalents section. Improved link text for accessibility.
• Troubleshooting page (new) -- Setup checklist and 10 documented failure scenarios with cause and fix for each, including SHA-1 mismatch, redirect_uri_mismatch, missing People API, token expiration on Web, and more.