feat(auth): Introduce resumable signin guide for the AmplifyJS

[joon-won]

Description of changes:

• This PR adds short introduction to a new feature for signInAPI in AmplifyJS, resumable signin

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
• 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>

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.

[joon-won]

Update: Freezing this PR as we're delaying introduction of ResumableSignIn feature

[mergify]

@joon-won, since a file was deleted from the src/pages and/or src/fragments directories, redirects might need to be set up so these previous pages do not 404. If redirects are needed, please answer these questions for each redirect that is needed:

• What is the source address of the redirect? (Where are you trying to redirect from?)

• What is the target address of the redirect? (Where are you trying to redirect to?)

• Type of redirect? 301 - permanent redirect or 302 - temporary redirect? (More info on Amplify Hosting redirects here: https://docs.aws.amazon.com/amplify/latest/userguide/redirects.html)