add auth example for federating with Google, Facebook, etc.

**Describe the content issue**:
A clear and concise description of what the content issue is. Is there content missing or is there incorrect content?

Revise and publish auth examples under `/[platform]/build-a-backend/auth/examples/`
https://github.com/aws-amplify/docs/blob/3e382bda677409f52c83fcf5cdaae6ea65cc300f/src/pages/%5Bplatform%5D/build-a-backend/auth/advanced-workflows/index.mdx?plain=1#L388

**URL page where content issue is**:

Start with the auth examples under `/[platform]/build-a-backend/auth/examples/` and read the referenced section in `src/pages/[platform]/build-a-backend/auth/advanced-workflows/index.mdx`. Revise and publish clear federation examples for Google, Facebook, and the other providers covered by the issue.

Written by the indexing model from the issue text.