feature(function): add custom functions documentation for AWS Amplify-backend

[MarlonJD]

… Gen 2 @aws-amplify/amplify-backend/1602

Description of changes:

Add custom runtime documents, it's required for #1602

Related GitHub issue

#1543, #1486

Instructions

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

• amplify-backend
• 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
amplify-backend

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[sobolk]

I left two comments around docker and go. But I suggest to wait until @josefaidt gets a chance to review this before making changes.

[josefaidt]

Hey @MarlonJD thank you for taking the time to write this up!! I left a few comments to adjust some of the language used but overall this looks great!

[MarlonJD]

Hi @josefaidt,
Thank you so much for your kind words and for taking the time to review this!
I really appreciate your feedback and have made adjustments based on your comments to refine the language. Please let me know if there’s anything else you'd like to tweak or improve further.