Fix docs webhooks.md

[Nittarab]

Description

The Webhooks.md guide is a bit inconsistent with the current library

1. as explained here #945 the WebhookHandler module need to extend the ShopifyAPI::Webhooks::Handler
2. path is a required argument of ShopifyAPI::Webhooks::Registry#add_registration

Checklist:

• [ ] My commit message follow the pattern described in here
• [ ] I have performed a self-review of my own code.
• [ ] I have added tests that prove my fix is effective or that my feature works.
• [ ] I have updated the project documentation.
• [ ] I have added a changelog line.

[Nittarab]

I signed the CLA but idk how to run this check again

[github-actions[bot]]

This pull request is stale because it has been open for 60 days with no activity. It will be closed if no further action occurs in 14 days.

[Nittarab]

@paulomarg do you want to take a look?

[Nittarab]

@mllemango Thank you. I have update the PR to remove the conflicts