Well, I looked at the page on negative numbers, and I'm afraid I'd find it more confusing than helpful if I were a student again:

"We have looked at unsigned numbers so far. In the computer, unsigned numbers start at zero and go to 2ⁿ-1. So, 2¹⁶ goes from 0 to 65535."

Whoa there!
You've just suddenly thrown in this 'n' from nowhere - what does it mean?

"2¹⁶ goes from 0 to 65535."

No, it doesn't.
2¹⁶ is exactly equal to 65536 - for all values of 2 and 16!

"To do that, we reserved the left-most bit to represent the sign of the number. Zero is a positive number and one is a negative number."

You could be clearer there; my first reaction was, No! one is not a negative number! - then I read it again (a couple of times) and realised that you mean "A zero in the sign bit indicates a positive number, and a one in the sign bit indicates a negative number."

"Example:

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   1011  decimal value is 11
   0100  1's complement
   0101  1's complement with 1 added = 2's complement = -11"

But you just said that a zero (in the left-most bit) indicates a positive number - so how can 0101₂ be negative?!

I think what you mean is::

   01011  decimal value is +11 (sign bit is zero)
   10100  1's complement
   10101  1's complement with 1 added = 2's complement = -11"

and then the example addition becomes:

     01011   decimal value +11
     10101+  decimal value -11
   ------- 
   1 00000