Document webpack change: allow to specify `publicPath` for each entrypoint

[webpack-bot]

A pull request by @sokra was merged and maintainers requested a documentation change.

See pull request: webpack/webpack#13137

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related to webpack-contrib/mini-css-extract-plugin#737

What kind of change does this PR introduce?
feature

Did you add tests for your changes?
yes

Does this PR introduce a breaking change?
no

What needs to be documented once your changes are merged?

• entry.xxx.publicPath allows to set the public path per entry (see also output.publicPath)

[anshumanv]

wip

[chenxsan]

Thank you for your interest in help, I'm assigning this issue to you now. However it would be great if you can finish it in maybe one week? I just want to make sure new issues are closed as soon as possible. If you can't make it, let me know and I'll revoke the assignment so someone else can take it over.