docs: replace Google OIDC setup with Microsoft Entra ID

[Arata1202]

Summary

• replace the Google-specific OIDC setup with Microsoft Entra ID instructions
• clarify that OPENID_CLIENT_SECRET uses the client secret Value, not the Secret ID
• update the Matrix callback instructions to reference the Entra ID app registration
• link to the upstream WorkAdventure documentation for other OIDC providers, including Google

Validation

• ran git diff --check
• verified the upstream WorkAdventure OIDC documentation link