Split in Group feature is only documented in release notes #6313
Description
See https://code.visualstudio.com/updates/v1_61#_split-an-editor-without-creating-a-new-group
I think it deserves a mention in https://code.visualstudio.com/docs/getstarted/userinterface#_side-by-side-editing and maybe also alongside https://code.visualstudio.com/docs/getstarted/tips-and-tricks#_side-by-side-editing