More tweaks to GCP setup instructions

[deflaux]

also

• now opening all external links in a new window
• silenced sphinx warnings about include files

The Bioc workshop material is still a work-in-progress so feel free to just skim that for now.

[mbookman]

Lots of very good stuff in here. Adding the screenshots will really help users. Two small comments. Otherwise LGTM.

[pgrosu]

I concur with Matt, the screenshots help a lot! I always try to include some in my docs (picture is worth ... :)

Thanks for the nice work! ~p

[deflaux]

thanks @mbookman! I'm about to start on a bunch of other feedback so I'll add port 22 in the next round.