Add docs on lighting

[asinghvi17]

CC @ffreyer - I'm not sure what shininess and specular do exactly, so I put some placeholder text in there for now.

[ffreyer]

To me specular kinda has a double meaning. The intended meaning is that it is a prefactor for the specular reflection (the bright reflection that sits between the light direction and the camera direction). The secondary meaning is that of a light color. If you were to set specular = Vec3f0(0.2, 0, 0) the specular reflection would be red.

Maybe it would make sense to have specular and light_color and convert this to one Vec3f0 in GLMakie.

shininess controls the exponent used in the calculation of the specular reflection. If shininess is large, the reflection will be sharp and local. If it's small, then the reflection is spans a wider area.

The relevant code for this is:

    vec3 H = normalize(L + V);
    float spec_coeff = pow(max(dot(H, N), 0.0), shininess);

    // final lighting model
    return vec3(
        ambient * color +
        diffuse * diff_coeff * color +
        specular * spec_coeff
    );

N is the normal vector. H = normalize(L + V) is the half vector between the light direction (lightposition - pos) and the camera direction (eyeposition - pos).

[SimonDanisch]

Awesome :)