Restructure documentation (#2114)

Breaks up the ARC documentation into several smaller articles. 

`@vijay-train` and `@martin389` put together the plan for this update, and I've just followed it here. 

In these updates:

- The README has been updated to include more general project information, and link to each new article.
- The `detailed-docs.md` file has been broken up into multiple articles, and then deleted.
- The Actions Runner Controller Overview doc has been renamed to `about-arc.md`.

Any edits to content beyond generally renaming headers or fixing typos is out of scope for this PR, but will be made in the future. 

Co-authored-by: Bassem Dghaidi <[email protected]>

docs/choosing-runner-destination.md

@@ -0,0 +1,91 @@
+# Adding ARC runners to a repository, organization, or enterprise
+
+## Usage
+
+[GitHub self-hosted runners can be deployed at various levels in a management hierarchy](https://docs.github.com/en/actions/hosting-your-own-runners/about-self-hosted-runners#about-self-hosted-runners):
+- The repository level
+- The organization level
+- The enterprise level
+
+Runners can be deployed as 1 of 2 abstractions:
+
+- A `RunnerDeployment` (similar to k8s's `Deployments`, based on `Pods`)
+- A `RunnerSet` (based on k8s's `StatefulSets`)
+
+We go into details about the differences between the 2 later, initially lets look at how to deploy a basic `RunnerDeployment` at the 3 possible management hierarchies.
+
+## Adding runners to a repository
+
+To launch a single self-hosted runner, you need to create a manifest file that includes a `RunnerDeployment` resource as follows. This example launches a self-hosted runner with name *example-runnerdeploy* for the *actions/actions-runner-controller* repository.
+
+```yaml
+# runnerdeployment.yaml
+apiVersion: actions.summerwind.dev/v1alpha1
+kind: RunnerDeployment
+metadata:
+  name: example-runnerdeploy
+spec:
+  replicas: 1
+  template:
+    spec:
+      repository: mumoshu/actions-runner-controller-ci
+```
+
+Apply the created manifest file to your Kubernetes.
+
+```shell
+$ kubectl apply -f runnerdeployment.yaml
+runnerdeployment.actions.summerwind.dev/example-runnerdeploy created
+```
+
+You can see that 1 runner and its underlying pod has been created as specified by `replicas: 1` attribute:
+
+```shell
+$ kubectl get runners
+NAME                             REPOSITORY                             STATUS
+example-runnerdeploy2475h595fr   mumoshu/actions-runner-controller-ci   Running
+
+$ kubectl get pods
+NAME                           READY   STATUS    RESTARTS   AGE
+example-runnerdeploy2475ht2qbr 2/2     Running   0          1m
+```
+
+The runner you created has been registered directly to the defined repository, you should be able to see it in the settings of the repository.
+
+Now you can use your self-hosted runner. See the [official documentation](https://help.github.com/en/actions/automating-your-workflow-with-github-actions/using-self-hosted-runners-in-a-workflow) on how to run a job with it.
+
+## Adding runners to an organization
+
+To add the runner to an organization, you only need to replace the `repository` field with `organization`, so the runner will register itself to the organization.
+
+```yaml
+apiVersion: actions.summerwind.dev/v1alpha1
+kind: RunnerDeployment
+metadata:
+  name: example-runnerdeploy
+spec:
+  replicas: 1
+  template:
+    spec:
+      organization: your-organization-name
+```
+
+Now you can see the runner on the organization level (if you have organization owner permissions).
+
+## Adding runners to an enterprise
+
+To add the runner to an enterprise, you only need to replace the `repository` field with `enterprise`, so the runner will register itself to the enterprise.
+
+```yaml
+apiVersion: actions.summerwind.dev/v1alpha1
+kind: RunnerDeployment
+metadata:
+  name: example-runnerdeploy
+spec:
+  replicas: 1
+  template:
+    spec:
+      enterprise: your-enterprise-name
+```
+
+Now you can see the runner on the enterprise level (if you have enterprise access permissions).