Перенос ветки для демо1 из док-ии Vega

src/guide/using-vue.md

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+# Using Vue in Markdown
+
+## Browser API Access Restrictions
+
+Because VuePress applications are server-rendered in Node.js when generating static builds, any Vue usage must conform to the [universal code requirements](https://ssr.vuejs.org/en/universal.html). In short, make sure to only access Browser / DOM APIs in `beforeMount` or `mounted` hooks.
+
+If you are using or demoing components that are not SSR friendly (for example containing custom directives), you can wrap them inside the built-in `<ClientOnly>` component:
+
+##