Tutorials: XGI in X minutes (#415)

* created xgi in 1 min tutorial

* toning down a bit the texts

* working progress XGI in 5 min

* XGI in 5 min

* wip XGI in 15 mins

* complete XGI in 15 minutes

* Addressed comments on tut. 5 minutes

* Addressed comments on XGI in 15 minutes

* added hyperlinks in XGI in 1 minute

* comments from max

* minor modifications to xgi in 1 minute

* minor modifications to xgi in 5 minutes

* minor modifications to xgi in 15 minutes