Better explain layers in Lens

Today in the Lens docs we barely talk about a central concept that is layers.
We mention it a few times, and document specific types of layers in dedicated sections such as annotations and reference lines, but we're not really explaining the concept or helping users understand it, how to use it more generally

We should make this clearer in the Lens docs. Maybe by:
- Adding a section about layers to explain what they are
- Listing the various types of layers available
- gathering existing sections about annotations and reference lines under that new layer section
- showing examples, or linking to relevant examples in the new chart docs

As a side quest to this, we should think about changing how we refer to "layer pane" in multiple places. We can surely find a better/clearer name to use, and replace those occurrences. By chatting with the visualization team, the `visualization editor` term was raised as a good option.