add README.md to interfaces

[vtavana]

In this PR,

1. Project README.md file is updated and a new README.md file is added to interfaces module. resolves top-level documentation for inverse real needs clarification #49
2. support for fftfreq, fftshift, ifftshift, and rfftfreq is added to both NumPy and SciPy interfaces by calling back on their original implementation in NumPy and SciPy. This is done for completeness as requested by fftshifts? #106
3. With this PR, To set mkl_fft as the backend for SciPy is only possible through mkl_fft.interfaces.scipy_fft as explained in the examples. resolves Stable API for scipy FFT backend #170

[Copilot]

Pull Request Overview

This PR extends the interfaces provided by mkl_fft by adding support for helper functions (fftfreq, fftshift, ifftshift, and rfftfreq) and updates the documentation and tests accordingly. It also refines the conda packaging workflow to ensure proper dependency installation.

• Added support for new FFT helper functions in both NumPy and SciPy interfaces.
• Introduced a dedicated README.md for the interfaces module and updated the root README and CHANGELOG.
• Adjusted tests and workflow definitions to match the new functionality and dependency requirements.

Reviewed Changes

Copilot reviewed 8 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
mkl_fft/tests/test_interfaces.py	Added tests to validate the existence of new FFT helper functions.
mkl_fft/interfaces/scipy_fft.py	Updated namespace imports and all to include new helper functions.
mkl_fft/interfaces/numpy_fft.py	Updated namespace imports and all to include new helper functions.
mkl_fft/interfaces/_scipy_fft.py	Removed legacy docstring and DftiBackend API definitions.
mkl_fft/interfaces/README.md	Added documentation for interfaces and usage examples.
README.md	Adjusted details and examples to reflect the new interfaces.
CHANGELOG.md	Updated change logs to reflect added functionality and modifications.
.github/workflows/conda-package.yml	Refined package creation and dependency installation commands.

Files not reviewed (1)

• mkl_fft/_pydfti.pyx: Language not supported

Comments suppressed due to low confidence (1)

mkl_fft/interfaces/_scipy_fft.py:63

• The removal of the detailed module docstring and the DftiBackend class in _scipy_fft.py may break backward compatibility for users relying on these API endpoints. Please consider adding appropriate deprecation warnings or updating the documentation to clearly indicate this breaking change.

Removed DftiBackend class and associated module docstring

[antonwolfy]

LGTM, few nits below