Fix postcss config: purgecss has been disabled for now

Because of the following error

```
Error: error building site: POSTCSS: failed to transform "main.css" (text/css): TypeError: purgecss is not a function
```

I commented out

https://github.com/ArkScript-lang/website/blob/master/config/postcss.config.js#L8-L64

Start by opening config/postcss.config.js and reproducing the site build that fails while transforming main.css. Check the commented purgecss section against the project's PostCSS and PurgeCSS setup; done means purgecss is enabled without the reported error and the site builds successfully.

Written by the indexing model from the issue text.