Webhooks guide

[izxxr]

Summary

This pull request adds the webhook guide. (#7662 was pointing towards master so had to create a new PR)

Feedback would be appreciated!

Checklist

• [ ] If code changes were made then they have been tested.
  • [ ] I have updated the documentation to reflect the changes.
• [ ] This PR fixes an issue.
• [ ] This PR adds something new (e.g. new method or parameters).
• [ ] This PR is a breaking change (e.g. methods or parameters removed/renamed)
• [x] This PR is not a code change (e.g. documentation, README, ...)

[Soheab]

This is missing the recently added client and bot_token kwargs.