[C++][Docs] Document `ARROW_SUBSTRAIT` CMake flag

### Describe the enhancement requested

The `ARROW_SUBSTRAIT` flag added in #31565 / #12915 is not included in the [list of CMake flags](https://arrow.apache.org/docs/dev/developers/cpp/building.html#optional-components) in the docs. Document it there.

### Component(s)

C++, Documentation