Update lambda triggers doc

[brandonwatson]

Description of changes:

I have updated the build-a-backend/storage/lambda-triggers documentation page based on my use of the functionality this past week as a developer. It was quite frustrating trying to use the docs as is to figure out how to modify the triggers, or even what was allowed.

As an solo-dev who is not super familiar with all of the cdk concepts which have been interwoven into gen 2 Amplify, I spent an inordinate amount of time trying to find a work around to the fact that the amplify FileUploader component uploads multiple files one at a time. If a customer is using this control in a modal dialog (for example) they are generally going to want an OK and CANCEL case. In my case, I wanted to trigger a lambda on OK. I went down a long and painful rabbit hole on how to trigger a lambda from the UX, and the prescribed method of adding a lambda on the schema actually breaks the backend build process if the lambda is being attached to storage. Then I went down the path of maybe having a REST API and spent hours on building that.

Before even testing it, I slept on it and thought more about how I might be able to accomplish what I want, which was to process all the files with my lambda in one go. This is a critical part of my specific use case. Triggering the lambda per file upload created race conditions etc. I solved this by uploading a manifest file from the dialog in my handleOK(). Having this file upload seemed like the right idea since there was a onUpload trigger. Sadly, the docs don't really explain that you can't do much in the way of modification to this trigger, which left me stuck again. I did more digging and eventually sorted out how to add an event notification via the backend.

This addition to the mdx would have saved me three days of coding.

FWIW, I cannot for the life of me figure out how to exclude all the files except the one mdx file change. Sorry!

Related GitHub issue #, if available:

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

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

• [ N] 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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[josefaidt]

Hey @brandonwatson 👋 thanks for taking the time to file this! It looks like a few yarn-specific config files were inadvertently committed, would you mind removing those? we can work with the team to add a packageManager key to prevent corepack from auto-adding one on install https://github.com/aws-amplify/docs/pull/8006/files#diff-7ae45ad102eab3b6d7e7896acd08c427a9b25b346470d7bc6507b6481575d519R122

[brandonwatson]

Hey @brandonwatson 👋 thanks for taking the time to file this! It looks like a few yarn-specific config files were inadvertently committed, would you mind removing those? we can work with the team to add a packageManager key to prevent corepack from auto-adding one on install https://github.com/aws-amplify/docs/pull/8006/files#diff-7ae45ad102eab3b6d7e7896acd08c427a9b25b346470d7bc6507b6481575d519R122

Updated with #8008