add auth docs

[josefaidt]

Description of changes:

internal branch follow-up to #7302 and #7308

• external identity providers
  • mobile snippets (signInWithRedirect)
• server-side document server-side auth #7355
  • how to validate user sessions
• managing users: console
  • prose
  • screenshots
• admin actions
  • prose
  • example code
• moving to production
  • prose for SES setup
  • prose for SMS
  • screenshots for SMS
• concepts/tokens and credentials
  • introductory information
  • mobile snippets
• concepts/user attributes (from https://next-docs.amplify.aws/react/build-a-backend/auth/manage-user-profile/#set-up-user-attributes)
  • basic usage examples
  • link out to "connect your frontend" for more
• user groups
  • understanding roles
  • basic examples for access control
• listen to auth events (hub)
  • mobile?
• manage user sessions
  • mobile examples
• using the authenticator -- this page is intentionally lean
  • other frameworks
  • mobile?
  • useAuthenticator hook (basic, link out to UI)
• customize auth lifecycle/email customization
  • with react.email document email customization examples with react.email, jsx.email #7351
  • with jsx.email document email customization examples with react.email, jsx.email #7351
• customize auth lifecycle/custom flows
• customize auth lifecycle/triggers
• example customize id token
• example customize access token
• manage accounts
• manage passwords
  • mobile examples
• advanced workflows/federation-only revise auth's advanced workflows > federation-only doc #7354
• clean up remaining mobile pages (most content should be ported in to structure)

Related GitHub issue #, if available:

Instructions

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

• amplify-cli
• amplify-ui
• amplify-studio
• amplify-hosting
• amplify-libraries

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

• JS
• Swift
• Android
• Flutter
• 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>

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