Document webpack change: Related assets

[webpack-bot]

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

See pull request: webpack/webpack#11254

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• add related info to asset info
• display assets in stats in a list instead of a table
• group related assets below the parent asset

What kind of change does this PR introduce?
feature

Did you add tests for your changes?
yes

Does this PR introduce a breaking change?
yes, stats output and format changed

What needs to be documented once your changes are merged?

• stats.relatedAssets allow to show related assets (they are hidden by default)

[anshumanv]

wip