Testimonials grid with semantic markup

This implementation uses a hybrid approach, on small screens the layout is flex-box and on large screens it's grid.

Note, the use of <q> for the marking up the testimonial messages, and how the document is structured using <article> with <section>'s and respective <header>'s, and a <footer> for the attribution.

This reduces reliance on classes, making the HTML more readable and maintainable.