Add dtype keyword argument to dpnp.fft.fftfreq and dpnp.fft.rfftfreq (#2384)

The PR proposes to align `dpnp.fft.fftfreq` and `dpnp.fft.rfftfreq`
functions with
[2024.12](https://data-apis.org/array-api/2024.12/extensions/generated/array_api.fft.fftfreq.html)
revision of Python array API spec where `dtype` keyword was specified
for the functions.

Additionally, implementation of `dpnp.fft.fftfreq` was reworked a bit to
consume less memory and to avoid allocating temporary arrays.
Also docstrings of FFT functions were improved to add blank lines prior
defaults values.

Author: antonwolfy

CHANGELOG.md

@@ -15,6 +15,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+* Extended `dpnp.fft.fftfreq` and `dpnp.fft.rfftfreq` functions to support `dtype` keyword per Python Array API spec 2024.12 [#2384](https://github.com/IntelPython/dpnp/pull/2384)
 * Allowed input array of `uint64` dtype in `dpnp.bincount` [#2361](https://github.com/IntelPython/dpnp/pull/2361)
 * The vector norms `ord={None, 1, 2, inf}` and the matrix norms `ord={None, 1, 2, inf, "fro", "nuc"}` now consistently return zero for empty arrays, which are arrays with at least one axis of size zero. This change affects `dpnp.linalg.norm`, `dpnp.linalg.vector_norm`, and `dpnp.linalg.matrix_norm`. Previously, dpnp would either raise errors or return zero depending on the parameters provided [#2371](https://github.com/IntelPython/dpnp/pull/2371)