expose solutions in markdown cells rather that code cells

[parmentelat]

hiya

I reckon this is relatively old stuff and that you have probably moved on, but:

while running this tuto recently, I found myself confused by the solution cells
having not realized they were code cells, I was happily running them without even realizing it

until I realized that the test cells were ALWAYS succeeding, which was suspicious
(being skilled is one thing, being invicible is another ;)

so, this version uses <details> to show solutions like this

Solution

in markdown cells to expose the solution but out of code
(in addition it's cool to be able to hide it again once you've taken a glimpse)

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This change is