vitepress/docs/.vitepress/theme/index.ts

// .vitepress/theme/index.js
import { h } from 'vue'
import DefaultTheme from 'vitepress/theme'
import AdComponent from './AdComponent.vue'

export default {
  ...DefaultTheme,
  Layout() {
    return h(DefaultTheme.Layout, null, {
      'doc-before': () => h(AdComponent)
    })
  }
}