feat(Auth): Update docs to reflect email MFA support

[osama-rizk]

Description of changes:

Updated documentation to reflect native email MFA support in Amplify backend. Removed outdated callouts stating email MFA is not supported with defineAuth and added configuration examples showing how to enable email MFA natively.

Changes made:

• Removed callout in MFA docs stating email MFA not supported with defineAuth
• Added email MFA configuration examples with email: true in multifactor settings
• Added callout about email MFA requirements (Plus/Essentials plan, SES configuration)
• Removed outdated CDK override section for email MFA workarounds
• Updated seed documentation to reflect native email MFA support

Related GitHub issue #, if available:

Addresses the completion of aws-amplify/amplify-backend#3036 which added native email MFA support.

Instructions

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

• amplify-cli

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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>

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