Document groups feature (#4578)
Document groups feature (#4578)

* Document groups feature

* Few edits, links additions

* typo - 'of'

switchinnovations/pxt,Microsoft/pxt,switchinnovations/pxt,switch-education/pxt,switchinnovations/pxt,switch-education/pxt,Microsoft/pxt,playi/pxt,Microsoft/pxt,playi/pxt,switch-education/pxt,switch-education/pxt,switch-education/pxt,Microsoft/pxt,switchinnovations/pxt,playi/pxt,playi/pxt,Microsoft/pxt