Document webpack change: fix: merge duplicate chunks

[webpack-bot]

A pull request by @alexander-akait was merged and maintainers requested a documentation change.

See pull request: webpack/webpack#18965

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What kind of change does this PR introduce?

fixes webpack/webpack#18898

Did you add tests for your changes?

WIP

Does this PR introduce a breaking change?

No

What needs to be documented once your changes are merged?

Now we export the plugin MergeDuplicateChunksPlugin, so developers can now use it with any stage and merge chunks after https://webpack.js.org/plugins/split-chunks-plugin/.

There is only one option - stage.