move content from slides to lesson episode

[wikfeldt]

for episode 8-multiple_gpu
if something in the slides is missing from the episode

also consider whether exercise code examples should be added into code boxes

[stepas-toliautas]

Also, speed of the Fortran exercises (Fortran not being that popular outside hardcore HPC) is too high for most participants. Some explaining before/ after/ around would help here as well.