title: Jenkinsfile nested stage(s) throwing error
body: <p>I have the following Jenkinsfile which I believe is setup correctly. I used <a href="https://jenkins.io/doc/book/pipeline/syntax/#sequential-stages" rel="noreferrer">https://jenkins.io/doc/book/pipeline/syntax/#sequential-stages</a> as an example but for some reason when I run this in jenkins I am receiving,</p>
  
  <blockquote>
    <p>WorkflowScript: 11: Unknown stage section "stages". Starting with
    version 0.5, steps in a stage must be in a steps block</p>
  </blockquote>
  
  <p>Can someone tell me what I am missing or doing wrong?</p>
  
  <pre class="lang-groovy prettyprint-override"><code>pipeline {
      agent {label 'windows'}
  
      stages {
          stage('Quick Build') {
              steps {
                  echo 'Building'
              }
          }
          stage('Deploy to Dev') {
              // when {
              //     branch 'develop' 
              // }
              stages {
                  stage('Building Distributable Package') {
                      steps {
                          echo 'Building'
                      }
                  }
                  stage('Archiving Package') {
                      steps {
                          echo 'Archiving Aritfacts'
                          archiveArtifacts artifacts: '/*.zip', fingerprint: true
                      }
                  }
                  stage('Deploying Dev') {
                      steps {
                          echo 'Deploying'
                          timeout(time:3, unit:'DAYS') {
                              input message: "Approve build?"
                          }
                      }
                  }
              }
  
          }
          stage('Deploy to Test') {
              when {
                  branch 'develop' 
              }
              steps {
                  echo 'deploying..'
                  timeout(time:3, unit:'DAYS') {
                      input message: "Approve build?"
                  }
              }
          }
          stage('Deploy to Prod') {
              when {
                  branch 'release' 
              }
              steps {
                  timeout(time:3, unit:'DAYS') {
                      input message: "Deploy to Prod?"
                  }
                  echo 'Deploying....'
              }
          }
      }
  }
  </code></pre>
  
  <p>Thanks in advance!</p>