feat(AEP-5): add AEP around designing an API

To help make the AEPs easier to consumer, as well as provide
a starting point for those who would like a from-basics starting
point on API design, provide a how-to guide.

aep/general/0005/aep.md

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+# Designing an API
+
+This AEP serves as a high-level guide on designing an API that adheres to the
+AEPs.
+
+## Process summary
+
+1. enumerate the user case would like your API to satisfy.
+1. identify resources and add them to your API.
+1. identify standard operations and add them to your API.
+1. identify custom operations and add them to your API.
+
+## Enumerate use cases
+
+The first step in designing an API is understanding precisely what you would
+like your user to be able to accomplish your API.
+
+Enumerate one or more use cases to clarify what precisel
+
+For example:
+
+- Create a VM.
+- List all VMs owned by a company.
+- List all VMs owned by a user.
+- Restart a running VM.
+- Trade an in-game item with another player.
+
+Some best practices:
+
+- Attempt to define granular use cases that can be composed to
+  satisfy more complex use cases.
+
+## Identify resources
+
+Once your use cases are defined, consider how many of those can be accomplished
+by represented by an [API Resource Type][]: similar to object-oriented
+programming, these are use cases that can be represented by entities that are
+created, read, updated, and deleted.
+
+Examples include:
+
+- users
+- virtual machine
+- load balancers
+- services
+
+The core of the AEPs is resource-oriented design: this design paradigm allows
+for uniform standard operations that reduce the cognitive overhead in learning
+about the operations and schemas exposed by your API.
+
+Resources can relate to each other, either as a parent-child relationship
+defining ownership, or via references that defines the resources usage of
+another resource in it's operations (e.g. a vm depending on a private network).
+
+See the following AEPs to learn more about resource-oriented design:
+
+- [resource paths][AEP-122]
+
+## Identify standard operations
+
+Once the resources are defined, identify one or more standard methods for each
+of those resources. Standard methods operate on the lifecycle of a resource
+lifecycle: namely, they create, read, update, delete, and list resources.
+
+Ideally all 5 standard methods should be exposed for every resource.
+
+See the following AEPs to learn more about the standard operations:
+
+- [AEP-131][]
+
+## Identify custom operations
+
+To accomplish some of the user journeys, resources may need to support more
+than just operations to create, read, update, delete, and list them. Some
+examples include:
+
+- restarting a virtual machine.
+- cleaning a disk.
+
+Break down each of the actions into granular operations, then follow the
+[custom operations][] AEP on how to design them.
+
+[API Resource Type]: /3#api-resource-type
+[custom operations]: /136

aep/general/0005/aep.yaml

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+---
+id: 5
+state: approved
+slug: intro
+created: 2024-01-10
+placement:
+  category: general