docs(plugin-nprogress): add plugin reference

Author: meteorlxy

docs/reference/plugin/nprogress.md

@@ -1,3 +1,15 @@
 # nprogress
 
-> TODO
+> @vuepress/plugin-nprogress
+
+Integrate [nprogress](https://github.com/rstacruz/nprogress) into VuePress, which can provide a progress bar when navigating to another page.
+
+## Styles
+
+You can customize the style of the progress bar via CSS variables:
+
+```css
+:root {
+  --nprogress-color: #29d;
+}
+```

docs/zh/reference/plugin/nprogress.md

@@ -1,3 +1,15 @@
 # nprogress
 
-> TODO
+> @vuepress/plugin-nprogress
+
+将 [nprogress](https://github.com/rstacruz/nprogress) 集成到 VuePress 中，在切换到另一个页面时会展示进度条。
+
+## 样式
+
+你可以通过 CSS 变量来自定义进度条的样式：
+
+```css
+:root {
+  --nprogress-color: #29d;
+}
+```