feat($core): Use mkdtemp for default tempdir

packages/@vuepress/core/lib/node/createTemp.js

@@ -1,4 +1,6 @@
 const { fs, path, chalk, logger } = require('@vuepress/shared-utils')
+const os = require('os')
+const process = require('process')
 
 /**
  * Create a dynamic temp utility context that allow to lanuch
@@ -12,7 +14,10 @@ const { fs, path, chalk, logger } = require('@vuepress/shared-utils')
 
 module.exports = function createTemp (tempPath) {
   if (!tempPath) {
-    tempPath = path.resolve(__dirname, '../../.temp')
+    tempPath = fs.mkdtempSync(path.join(os.tmpdir(), 'vuepress-'))
+    process.on('exit', _code => {
+      fs.removeSync(tempPath)
+    })
   } else {
     tempPath = path.resolve(tempPath)
   }

packages/docs/docs/config/README.md

@@ -69,10 +69,10 @@ Specify the port to use for the dev server.
 ### temp
 
 - Type: `string`
-- Default: `/path/to/@vuepress/core/.temp`
+- Default: `/tmp/vuepress-SUFFIX`
 
 
-Specify the temporary directory for client.
+Specify the temporary directory for client. `SUFFIX` is a random suffix generated by `fs.mkdtemp`.
 
 ### dest
 
@@ -321,7 +321,7 @@ This option is also included in [Plugin API](../plugin/option-api.md#extendmarkd
 - Default: `['h2', 'h3']`
 
 While preparing the page, headers are extracted from the Markdown file and stored in `this.$page.headers`. By default, VuePress will extract `h2` and `h3` elements for you. You can override the headers it pulls out in your `markdown` options.
- 
+
 ``` js
 module.exports = {
   markdown: {

packages/docs/docs/zh/config/README.md

@@ -67,9 +67,9 @@ module.exports = {
 ### temp
 
 - Type: `string`
-- Default: `/path/to/@vuepress/core/.temp`
+- Default: `/tmp/vuepress-SUFFIX`
 
-指定客户端文件的临时目录。
+指定客户端文件的临时目录。`SUFFIX` 是 `fs.mkdtemp` 产生的随机后缀。
 
 ### dest
 
@@ -317,7 +317,7 @@ module.exports = {
 - 默认值: `['h2', 'h3']`
 
 Markdown 文件的 headers (标题 & 小标题) 会在准备阶段被提取出来，并存储在 `this.$page.headers` 中。默认情况下，VuePress 会提取 `h2` 和 `h3` 标题。你可以通过这个选项来修改提取出的标题级别。
- 
+
 ``` js
 module.exports = {
   markdown: {