Update 'Defining blocks' page a bit (#5257)
Update 'Defining blocks' page a bit (#5257)

* local commit

* add new sections

* 'and' to 'an'

switch-education/pxt,switch-education/pxt,Microsoft/pxt,Microsoft/pxt,Microsoft/pxt,Microsoft/pxt,Microsoft/pxt,switch-education/pxt,switch-education/pxt,switch-education/pxt