feat(3): add terminology for root resources and collections

We are missing a term to describe a resource whose collection has
no parents. Since this is helpful for things like building a navigation
tree, define the term.

aep/general/0003/aep.md.j2

@@ -80,6 +80,12 @@ addresses.
 Either a programmatic client or a user that consumes an API. This term should
 be used when a statement refers broadly to both programs and users.
 
+### Collection
+
+A collection represents a discrete set of a specific resource type. A resource
+may have one or more collections, with each collection having it's own set of
+parent collections.
+
 ### Declarative Clients
 
 Declarative Clients, also known as Infrastructure as Code (IaC), describes a
@@ -99,6 +105,22 @@ Examples of complexities that declarative clients abstract away include:
 
 [Terraform][] is an example of such a client.
 
+### Root collection
+
+A [collection](#collection) that does not have a parent collection. For example,
+`/users/`.
+
+### Root Resource
+
+A `root` resource is a resource which has a collection that has no parents. In
+other words, it is a resource that has a collection that appears at the root of
+a resource hierarchy.
+
+A root resource may also appear as a child of another resource: for example, a
+user permission may exist globally, but also as a child of a individual object.
+In this case, the resource is still considered a "root" resource, because at
+least one collection does not have a parent.
+
 ### Schema
 
 A schema describes the structure of the request or response of an [API

aep/general/0121/aep.md.j2

@@ -28,9 +28,10 @@ When designing an API, consider the following:
 A resource-oriented API **must** be modeled as a resource hierarchy, where each
 node is either a simple resource or a collection of resources.
 
-A _collection_ contains resources of _the same type_. For example, a publisher
-has the collection of books that it publishes. A resource usually has fields,
-and resources may have any number of sub-resources (usually collections).
+A [_collection_](./0003#collection) contains resources of _the same type_. For
+example, a publisher has the collection of books that it publishes. A resource
+usually has fields, and resources may have any number of sub-resources (usually
+collections).
 
 **Note:** While there is some conceptual alignment between storage systems and
 APIs, a service with a resource-oriented API is not necessarily a database, and