GH-44903: [C++] Add the Expm1(exponent) scalar arithmetic function (#44904)

`Expm1(exponent)` is a more accurate way of computing `Exp(exponent) - 1.0` for small values of exponent.

### Rationale for this change

`expm1(x)` is specifically designed to compute `exp(x)−1` more accurately, particularly for small x. It uses numerical techniques and approximations that minimize the loss of precision.

When x is very small (close to 0), `exp(x)` is approximately `1+x`. Subtracting 1 from `exp(x)` (i.e., `(exp(x)−1)` can result in significant cancellation of significant digits due to floating-point arithmetic, leading to a loss of precision).

For example:

When `x = 10^−8`, `exp(x)` is close to `1 + 10^−8`, so subtracting 1 leaves only the small `10^−8`, which may lose accuracy due to floating-point limitations.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes and documentation was updated to list the new function.
* GitHub Issue: #44903

Authored-by: Felipe Oliveira Carvalho <felipekde@gmail.com>
Signed-off-by: Felipe Oliveira Carvalho <felipekde@gmail.com>