Add dtFFT package to index

[ShatrovOA]

This PR adds https://github.com/ShatrovOA/dtFFT/ to the package index.

Required

• Relevance - dtFFT is a high-performance library designed for parallel data transpositions and optional Fast Fourier Transforms (FFTs) in multidimensional computing environments.
• Maturity - The code is stable, with regular updates, and the repository is actively developed.
• Availability - The source code is freely available on GitHub.
• Open source - The package is licensed under the GPL-3.0 License, ensuring it remains free and open-source.
• Uniqueness - This is an original repository, not a fork.
• README - The README file exists and clearly states the purpose of the project. Installation steps, building the code, and running the tests are detailed in the documentation.

Recommended

• Documentation - Consists of two parts: user-friendly and auto-generated via FORD.
• Contributing - Ways of contributing are listed in the README file.
• Tests - Available for all provided interfaces: Fortran, C, and C++.