Add new apps

We have added info for the following new apps.
- A benchmark framework for evaluating general-purpose, i.e., universal, machine learning potentials, along with a leaderboard based on those evaluations, “Matbench Discovery“
- Software for first-principles calculations of electron-phonon interactions, “EPW (Electron-Phonon Wannier)“
- A graphical user interface (GUI) program for pre- and post-processing with the DFT package SIESTA, “GUI4dft“