refactor: Move API Docs under References

[stocaaro]

Description of changes:

This change moves our automated reference docs from being located under their related category to being consolidated under a single subdirectory in the "Reference" menu heading.

• The content remains the same.
• The heading changes to reflect the import path/source.
• Redirects added from all of the old urls to the new urls.

This change is being made to make the existing api docs more usable by associating them with their import source and allow support for reference docs that aren't one to one associated between export surface and category (already true for aws-amplify/api and more true for @aws-amplify/backend)

Screenshots from change

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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.

[mergify]

@stocaaro, 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)