Argument implicitness: differentiable{Within}At_zero

Stream: mathlib4

[Michael Rothgang]

Today's inconsistency in mathlib: docs#differentiableAt_zero has x an explicit argument, while docs#differentiableWithinAt_zero has s and x implicit. Which way do we prefer?

[Michael Rothgang]

Bonus points for making the mdifferentiableAt versions consistent with that, and the continuousAt versions also.