fix(Authentication): Keep social sigin-in page uniform across all pla…

[ashwinkumar6]

Description of changes:

1. Keep social sigin page uniform across all platforms

`Authentication -> Social sign-in (OAuth)` is only available for Js
`Authentication -> Social sign in with web UI` is only available for iOS, Android and Flutter

Merge these under a common title
`Authentication -> Social sign-in (OAuth)` is available for all platforms (JS, iOS, Android and Flutter)

The following URLs are replaced
- FROM `lib/auth/social_signin_web_ui/q/platform/ios/` TO  `lib/auth/social/q/platform/ios/`
- FROM `lib/auth/social_signin_web_ui/q/platform/android/` TO  `lib/auth/social/q/platform/android/`
- FROM `lib/auth/social_signin_web_ui/q/platform/flutter/` TO  `lib/auth/social/q/platform/flutter/`

2. Add more detailed description to Social sign-in (OAuth) using JS page Detail out the Amplify CLI amplify add auth steps

3. Mention the possibility of amplify update auth for iOS, Android, Flutter pages

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.