Document z-index layers

[szuperaz]

The SDK uses z-index inside a few components. When an integrator created a custom template they sometimes have to set z-index too. Their settings have to be compatible with SDK z-index settings. This is hard at the moment, as we have no documentation about z-index usage.

To solve this issue we should create variables for z-index settings:

--layer-100: 100;

.elementX {
    z-index: var(--layer-100);
}

.elementY {
    z-index: var(--layer-100);
}

We should document those variables and list where they are used in the code -> our current CSS variable generation workflow doesn't support this out of the box, but can be updated to suit this use-case