docs(auth): update to include email mfa

[jjarvisp]

Description of changes:

The purpose of this pull request is to update documentation to include Email MFA usage.

Staging Link

The majority of updates are to the following pages:

• Multi-Factor Authentication
• Sign Up
• Sign In
• Multi Step Sign In

Related GitHub issue #, if available:

Instructions

If this PR should not be merged upon approval for any reason, please submit as a DRAFT

Which product(s) are affected by this PR (if applicable)?

• [ ] amplify-cli
• [ ] amplify-ui
• [ ] amplify-studio
• [ ] amplify-hosting
• [X] amplify-libraries

Which platform(s) are affected by this PR (if applicable)?

• [X] JS
• [X] Swift
• [X] Android
• [X] Flutter
• [X] React Native

Please add the product(s)/platform(s) affected to the PR title

Checks

• [ ] Does this PR conform to the styleguide?

• [ ] Does this PR include filetypes other than markdown or images? Please add or update unit tests accordingly.

• [ ] Are any files being deleted with this PR? If so, have the needed redirects been created?

• [ ] Are all links in MDX files using the MDX link syntax rather than HTML link syntax?
  ref: MDX: [link](https://docs.amplify.aws/) HTML: <a href="https://docs.amplify.aws/">link</a>

When this PR is ready to merge, please check the box below

• [ ] Ready to merge

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