argocd: Deploy Argo CD

[Argo CD] is a Kubernetes-native GitOps/continuous deployment manager.
It monitors the state of Kubnernetes resources, such as Pods,
Deployments, ConfigMaps, Secrets, and Custom Resources, and synchronizes
them with their canonical definitions from a Git repository.

*Argo CD* consists of various components, including a Repository
Service, an Application Controller, a Notification Controller, and an
API server/Web UI.  It also has some optional components, such as a
bundled Dex server for authentication/authorization, and an
ApplicationSet controller, which we will not be using.

[Argo CD]: https://argo-cd.readthedocs.io/

argocd/README.md

@@ -0,0 +1,46 @@
+# Argo CD
+
+> [Argo CD] is a declarative GitOps continuous delivery tool, which allows
+> developers to define and control deployment of Kubernetes application
+> resources from within their existing Git workflow.
+
+```sh
+kubectl apply -k argocd
+kubectl apply -f argocd/applications
+```
+
+## Components
+
+Argo CD consists of several components, some of which are not used:
+
+- [x] Application Controller
+- [x] Repository Service
+- [x] Web Server
+- [x] Notification Controller
+- [ ] ApplicationSet Controller[^1]
+- [ ] Dex Server[^2]
+
+[^1]: ApplicationSets are "generators" that can be used to apply applications
+    to multiple clusters.  As we only have a single cluster, it is not useful.
+[^2]: Argo CD includes Dex to handle authentication and authorization, but we
+    are using Authelia instead.
+    
+
+## Applications
+
+*Applications* are the core resource in Argo CD.  They form a collection of
+resources associated with a particular application deployment.  They are
+themselves defined as Kubernetes resources (see [applications]).
+
+
+## Git Webhook
+
+*Argo CD* will automatically refresh the desired state of applications whenever
+a changeset is pushed to the Git repository where manifests are stored.  The
+[infra/kubernetes] repository has a Webhook configured in Gitea that notifies
+the Argo CD server on Git push events.
+
+
+[Argo CD]: https://argo-cd.readthedocs.io/
+[applications]: ./applications/
+[infra/kubernetes]: https://git.pyrocufflink.blue/infra/kubernetes