body {
    margin: 0;
    padding: 0;
}
#site {
    display: grid;
    grid-template-columns: repeat(2, 1fr);
    grid-template-areas: 
                    "gauche droite";
}
#gauche {
    background-image: url(image.jpg);
    background-size: cover;
    background-position: center;
    position: fixed;
    width: 50%;
    height: 100%;
    grid-area: gauche;
}
#droite {
    grid-area: droite;
    padding: 20px;
    font-family: sans-serif;
    font-size: 18px;
    background-color: #f3f3f3;
}
#droite h1, #droite h2 {
    text-align: center;
}
#droite p {
    text-align: justify;
}

@media screen and (max-width: 900px) {
    #site {
        grid-template-columns: 1fr;
        grid-template-areas: 
                        "gauche"
                        "droite";
    }
    #gauche {
        width: 100%;
        height: 50%;
    }
    #droite {
        position: absolute;
        top: 50%;
    }
}