Docs: add image placeholder callouts throughout guides

[silverbackdan]

Task

As we write and refine the getting-started guides, we should add placeholder callouts at each point where a screenshot or diagram would help:

• Admin panel showing the layout/page/component in edit mode
• The resource chain diagram (layouts → pages → component groups → components)
• The admin create forms for Layout, Page, etc.
• Edit mode showing a CwaComponentGroup region highlighted

Each placeholder should describe what the image should show, so it's clear for whoever creates the visuals.

Format suggestion

::placeholder-image{label="IMAGE: Admin panel showing layout tab with navigation component group in edit mode"}
::

Or a standard callout block with an image icon and the description.

Idea from internal docs review (nice-to-have).