[SYCL][Doc] Less verbose name for proposed FP4,8 (#21302)
Update the class names for all the FP8 and FP4 types to follow this
pattern:
```
template<size_t N>
class fp8_e4m3_x {/*...*/};
using fp8_e4m3 = fp8_e4m3_x<1>;
using fp8_e4m3_x2 = fp8_e4m3_x<2>;
```
This is less verbose in the common case where the user knows they want
either 1 or 2 elements. For anyone writing generic code, where the
number of elements is a parameter, you can still use the `fp8_e4m3_x<N>`
form.
