老師請問可以講解一下這邊目錄渲染的邏輯嗎?搞不太明白這個flatNavigation的作用，還有這個doFlatNavigation的作用

 <div class="book-detail-content-item" v-for="(item, index) in flatNavigation" :key="index" @click="read(item)">
              <div class="book-detail-content-navigation-text"
                   :class="{'is-sub': item.deep> 1}"
                   :style="itemStyle(item)"
                   v-if="item.label">{{item.label}}
              </div>
</div>

      flatNavigation() {
        if (this.navigation) {
          return Array.prototype.concat.apply([], Array.prototype.concat.apply([], this.doFlatNavigation(this.navigation.toc)))
        } else {
          return []
        }
       },
 
      doFlatNavigation(content, deep = 1) {
        const arr = []
        content.forEach(item => {
          item.deep = deep
          arr.push(item)
          if (item.subitems && item.subitems.length > 0) {
            arr.push(this.doFlatNavigation(item.subitems, deep + 1))
          }
        })
        return arr
      },