Open docs in new tab (#3427)
Open docs in new tab (#3427)

* Open docs in new page rather than the side docs.

* Only load in url if it starts with a slash.

* Show a warning

* Fix else case

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