docs: plugins documentation placeholder

packages/@vuepress/markdown/README.md

 # @vuepress/markdown
 
 > markdown for vuepress
+
+## Usage
+
+```javascript
+const createMarkdown = require('@vuepress/markdown')
+const md = createMarkdown(/* options */)
+```
+
+## Options
+
+### slugify
+
+- TODO
+
+### externalLinks
+
+- TODO
+
+### anchor
+
+- TODO
+
+### toc
+
+- TODO
+
+### lineNumbers
+
+- TODO
+
+### beforeInstantiate
+
+- TODO
+
+### afterInstantiate
+
+- TODO

packages/@vuepress/plugin-back-to-top/README.md

@@ -6,7 +6,7 @@
 
 ```javascript
 module.exports = {
-  plugins: ['back-to-top'] 
+  plugins: ['@vuepress/back-to-top'] 
 }
 ```
 

packages/@vuepress/plugin-blog/README.md

 # @vuepress/plugin-blog
 
 > theme plugin for vuepress
+
+## Usage
+
+```javascript
+module.exports = {
+  plugins: ['@vuepress/blog'] 
+}
+```
+
+## Options
+
+### pageEnhancers
+
+- Type: `function`
+- Default: `/i18n/`
+
+### categoryIndexPageUrl
+
+- Type: `string`
+- Default: `/category/`
+
+### tagIndexPageUrl
+
+- Type: `string`
+- Default: `/tag/`
+
+
+

packages/@vuepress/plugin-google-analytics/README.md

@@ -2,6 +2,14 @@
 
 > google-analytics plugin for vuepress
 
+## Usage
+
+```javascript
+module.exports = {
+  plugins: ['@vuepress/google-analytics'] 
+}
+```
+
 ## Options
 
 ### ga

packages/@vuepress/plugin-i18n-ui/README.md

@@ -2,7 +2,15 @@
 
 > i18n-ui plugin for vuepress
 
-## Plugin Options
+## Usage
+
+```javascript
+module.exports = {
+  plugins: ['@vuepress/i18n-ui'] 
+}
+```
+
+## Options
 
 ### route
 

packages/@vuepress/plugin-last-updated/README.md

@@ -4,6 +4,14 @@
 
 > Note that this plugin has been included in the core.
 
+## Usage
+
+```js
+module.exports = {
+  plugins: ['@vuepress/last-updated'] 
+}
+```
+
 ## Options
 
 ### transformer

packages/@vuepress/plugin-medium-zoom/README.md

 # @vuepress/plugin-medium-zoom
 
 > medium-zoom plugin for vuepress
+
+## Usage
+
+```javascript
+module.exports = {
+  plugins: ['@vuepress/medium-zoom'] 
+}
+```

packages/@vuepress/plugin-pagination/README.md

@@ -2,3 +2,32 @@
 
 > pagination plugin for vuepress
 
+## Usage
+
+```javascript
+module.exports = {
+  plugins: ['@vuepress/pagination'] 
+}
+```
+
+## postsFilter
+
+- Type: `function`
+- Default:
+
+```js
+(({ type }) => type === 'post')`
+```
+
+## postsSorter
+
+- Type: `function`
+- Default:
+
+```js
+((prev, next) => {
+  const prevTime = new Date(prev.frontmatter.date).getTime()
+  const nextTime = new Date(next.frontmatter.date).getTime()
+  return prevTime - nextTime > 0 ? -1 : 1
+})
+```

packages/@vuepress/plugin-pwa/README.md

@@ -2,6 +2,14 @@
 
 > PWA plugin for vuepress
 
+## Usage
+
+```javascript
+module.exports = {
+  plugins: ['@vuepress/pwa'] 
+}
+```
+
 ## Options
 
 ### serviceWorker

packages/@vuepress/plugin-search/README.md

@@ -39,4 +39,4 @@ import SearchBox from '@SearchBox'
 - Type: `number`
 - Default: `true`
 
-Set the maximum number of results for search
+Set the maximum number of results for search.

packages/@vuepress/theme-default/README.md

@@ -6,5 +6,5 @@
 
 The default theme has the following plugin built in:
 
-- @vuepress/plugin-active-header-links
-- @vuepress/plugin-google-analytics
+- [@vuepress/plugin-active-header-links](https://github.com/vuejs/vuepress/tree/master/packages/@vuepress/plugin-active-header-links)
+- [@vuepress/plugin-google-analytics](https://github.com/vuejs/vuepress/tree/master/packages/%40vuepress/plugin-google-analytics)
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