Document webpack change: add ChunkGraph

[webpack-bot]

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

See pull request: webpack/webpack#7889

---

• chunk modules and module chunks are no longer sorted while compiling. Instead they are sorted when requesting it with order.
• connectChunkAndModule, disconnectChunkAndModule have moved from GraphHelpers into ChunkGraph
• moved common comparator functions into util/comparators
• removed modules from Chunk
• removed chunks from Module
• add Compilation.chunkGraph
• improve format-file-header tooling to support generics

What kind of change does this PR introduce?
refactoring

Did you add tests for your changes?
existing tests

Does this PR introduce a breaking change?
yes

What needs to be documented once your changes are merged?
Many methods in Module and Chunk are moved to ChunkGraph