Fix vuepress compilation memory problem修复vuepress编译内存问题

[SailVR]

• I confirm that this is an issue rather than a question.

Bug report

Steps to reproduce

What is expected?

What is actually happening?

编译vuepress时，JavaScript显示内存不够，是不是vuepress多线程编译所以占用内存太多，我用0.x版本的vuepress编译就没有任何问题，希望官方可以修复下
When compiling vuepress, JavaScript shows that there is not enough memory. Is it due to vuepress multi-threaded compilation, so it takes up too much memory. I have no problem with the 0.x version of vuepress compilation. I hope the official can fix it.

Other relevant information

• Output of npx vuepress info in my VuePress project:

[kefranabg]

How many pages are you trying to build? Do you have a reproduction repo?

[jixuan1989]

Hi @kefranabg , we have 174 files totally, and it can be reproduced.

The codes are at https://github.com/apache/incubator-iotdb/tree/master/site
and the markdowns are at https://github.com/apache/incubator-iotdb/tree/master/docs

[th0r]

You can try --max-concurrency option - it helped me reduce memory usage during a build from ~8G to ~3.5G.