make `postcss-loader` official

[alexander-akait]

Feature to document

make postcss-loader official (now it under webpack-contrib) https://github.com/webpack-contrib/postcss-loader

Author(s)

No

Additional information

No

[ ] I am willing to work on this issue and submit a pull request.

[anshumanv]

wip

[anshumanv]

I think already official

[alexander-akait]

@anshumanv I think we need check url of postcss-loader in our docs, because it migrate to other organization

[anshumanv]

Already - #3909 😄

[alexander-akait]

Great