`ColumnGroup` description should link to `group {}` operation

[Jolanrensen]

https://kotlin.github.io/dataframe/datacolumn.html#columngroup tells nothing about how to create a column group.

It should probably link to https://kotlin.github.io/dataframe/group.html or split-inward

[Jolanrensen]

@jetbrains-junie maybe?

[jetbrains-junie]

Hey, it’s Junie by JetBrains! I started processing your request 🚀

[jetbrains-junie]

✅ The execution was successful

The "ColumnGroup" description in the documentation was successfully updated to include links to the group operation and split inward guidance. This change addresses the initial issue of lacking information on how to create a column group, aligning with the specified requirements. No errors occurred during the process.

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