Example: Unpacking with subscriptors

[steffen19w23]

Since the result of

a=[1,0]; a[0],a[a[0]]=0,a[0]; print(a)

was unexpected for me, I prepared a section.

[steffen19w23]

@satwikkansal no comment?

[satwikkansal]

@steffen19w23 Thanks for your PR.

I loved the example, but, I see some scope of improvement in simplification and presentation. That's why I've left it as is, I'll make minor modifications in your PR and merge it when I do that next major revision. Hope that's okay.