feat(docs): Cookbook Onboard (AI Assistant) integration

[ClockRide]

What I Did

Extended custom.js with code to instantiate the Cookbook Onboard integration.

Resolves: #2197

How to Verify It

Launch the project in production or a dev environment and try out Cookbook Onboard by clicking the "Ask Cookbook" button in the bottom right corner of the docs :)

You can also use following preview url: https://apeworx-cookbook.vercel.app/ape/latest/userguides/quickstart.html

Checklist

• [x] All changes are completed
• [ ] New test cases have been added
• [x] Documentation has been updated