refactor dev docs and add dtype details during model building

docs/dev/dev.rst

@@ -3,6 +3,6 @@ Developer Guide
 
  .. toctree::
 
-    extending
     contributing
-    tools
+    under_the_hood
+    extending

docs/dev/under_the_hood.rst

@@ -0,0 +1,23 @@
+Under The Hood
+##############
+
+This page explains key aspects of the infrastructure.
+
+Model Building
+==============
+
+
+Model Builder Decorator
+-----------------------
+The following decorator should be used for new model builders, e.g. ::
+
+  @nequip.model.model_builder
+  def my_new_model_builder(arg1, arg2):
+      return model(arg1, arg2)
+
+
+.. autofunction:: nequip.model.model_builder
+
+Dtypes in Model Building
+------------------------
+``model_dtype`` is imposed by using ``torch.set_default_dtype()`` to set the default ``dtype`` to ``model_dtype`` for the duration of model building, such that parameter tensors created during model building will be in the default ``dtype``, which was set to ``model_dtype``.