storage tutorial improvements

[rkdarst]

The storage tutorial could use some better exercises, less technical and abstract. We were thinking that it could be more about lightweight testing of real code to understand how much i/o it does.

• benchmark a program, e.g. import python -e file-c 'import keras'
• strace is simplest
• understand i/o of a file, strace -e file
• ... ?

Some of this could be combined with the profiling page.

[tpfau]

I think some of this has been done but probably can still be improved