Update docs with details about stack details in `pull_request` webhook event

[willsmythe]

Summary

pull_request webhook events now include a stack object if the PR is in a stack. This object currently includes information about the base of the stack (the name of the branch the stack is targeting and the current SHA of that branch):

{
   pull_request: {
     stack: {
       base: {
         ref: "main"
         sha: "blah"
       }
     }
  }
}

Update the docs to include this information. This will be particularly helpful for teams needing to trigger CI or workflows on any PR in a stack that targets main for example.