[doc] Add documentation about layer-under-layer errors and how to mitigate

It is illegal in FIRRTL to have layers nested under other layers across an instantiation hierarchy. These errors are difficult to address, though the tools exist (`layer.elideBlocks`, `layer.enable`, and the boolean parameters to the `layer.block` API). However, this is all not documented or not coherently documented.

Add to the existing Layers website doc to show these kinds of errors and the different approaches to fix them.

Start by locating the existing Layers website documentation and review how layers are currently explained. Document layer-under-layer errors across instantiation hierarchies and show mitigation with layer.elideBlocks, layer.enable, and the boolean parameters of layer.block; done means the approaches are explained coherently with examples.

Written by the indexing model from the issue text.