You can separate category names with periods ( . ) when you call Configuration#get to create nested categories, e.g. config.get("general.nested_general", "property_in_nested", 2)
The Forge documentation explains how to set up a workspace here. It explains some of the basics about Blocks here and Items here. Registering things like Blocks and Items is explained here.
If you have any specific questions, you can create a topic in the Modder Support section.
Your model extends from builtin/entity, which means Minecraft expects it to be rendered by a TileEntitySpecialRenderer. Since you don't have one, nothing renders.
Ideally you should use a regular baked model for the shield rather than a TESR.