Document the output directory for devServer.writeToDisk

[bseib]

Regarding the webpack-dev-server documentation for devServer.writeToDisk, the docs would be improved (especially for newcomers) if we were told where to find the files that get written to disk.

I think I figured out that the files go to ./dist. However I'm not sure if that is true for everyone, or if that's just for my own setup. The docs should tell the reader exactly what to expect. I'll make a PR if the answer is always ./dist, or if someone tells me what the correct answer is.

FYI, the current documentation for devServer.writeToDisk looks like this:

https://webpack.js.org/configuration/dev-server/#devserverwritetodisk-