Update docs for SMS and requesting an origination number, has moved from Pinpoint/SMS over to AWS End User Messaging

[chrisl777]

Describe the content issue:

The content below is now outdated due to phone number and SMS management moving over to AWS End User Messaging.

In addition, the instructions here are quite terse. It is difficult to understand the exact steps needed to enable SMS for auth flows with Amplify.

In order to send SMS authentication codes, you must [request an origination number](https://docs.aws.amazon.com/pinpoint/latest/userguide/settings-request-number.html). Authentication codes will be sent from the origination number. If your AWS account is in the SMS sandbox, you must also add a destination phone number, which can be done by going to the [Amazon Pinpoint Console](https://console.aws.amazon.com/pinpoint/), selecting SMS and voice in the navigation pane, and selecting Add phone number in the Destination phone numbers tab. To check if your AWS account is in the SMS sandbox, go to the [SNS console](https://console.aws.amazon.com/sns/), select the Text messaging (SMS) tab from the navigation pane, and check the status under the Account information section.

URL page where content issue is:

Live link:
https://docs.amplify.aws/react/build-a-backend/auth/moving-to-production/#sms

Repo link:

docs/src/pages/[platform]/build-a-backend/auth/moving-to-production/index.mdx

Line 94 in 305b960

In order to send SMS authentication codes, you must [request an origination number](https://docs.aws.amazon.com/pinpoint/latest/userguide/settings-request-number.html). Authentication codes will be sent from the origination number. If your AWS account is in the SMS sandbox, you must also add a destination phone number, which can be done by going to the [Amazon Pinpoint Console](https://console.aws.amazon.com/pinpoint/), selecting SMS and voice in the navigation pane, and selecting Add phone number in the Destination phone numbers tab. To check if your AWS account is in the SMS sandbox, go to the [SNS console](https://console.aws.amazon.com/sns/), select the Text messaging (SMS) tab from the navigation pane, and check the status under the Account information section.

[josefaidt]

Hey @chrisl777 👋 thanks for raising this! We'll get that link updated and add some color to the setup docs.