Add documentation for `css.extract` object configuration

[Etheryte]

What problem does this feature solve?

Currently, the docs for css.extract state that it's of type boolean | Object.
However, valid values for the object-based configuration are not explained nor are there any details on where to look for further info.

I'd assume the library used is mini-css-extract-plugin, but right now it isn't clear from the documentation.

What does the proposed change look like?

Please either add a link to external documentation or add a brief description inline.

[NataliaTepluhina]

Closing this as related PR was merged.