Explain more up-front in website homepage

This commit

 - adds a brief explanation of what Flux does
 - makes the target features a little more self-explanatory
 - gives GitOps Toolkit its own index page

Signed-off-by: Michael Bridgen <michael@weave.works>

docs/dev-guides/index.md

@@ -0,0 +1,27 @@
+# GitOps Toolkit
+
+The GitOps Toolkit is the set of APIs and controllers that make up the
+runtime for Flux v2. The APIs comprise Kubernetes custom resources,
+which can be created and updated by a cluster user, or by other
+automation tooling.
+
+You can use the toolkit to extend Flux, and to build your own systems
+for continuous delivery. The [the source-watcher
+guide](https://toolkit.fluxcd.io/dev-guides/source-watcher/) is a good
+place to start.
+
+A reference for each component is linked below.
+
+- [Source Controller](../components/source/controller.md)
+    - [GitRepository CRD](../components/source/gitrepositories.md)
+    - [HelmRepository CRD](../components/source/helmrepositories.md)
+    - [HelmChart CRD](../components/source/helmcharts.md)
+    - [Bucket CRD](../components/source/buckets.md)
+- [Kustomize Controller](../components/kustomize/controller.md)
+    - [Kustomization CRD](../components/kustomize/kustomization.md)
+- [Helm Controller](../components/helm/controller.md)
+    - [HelmRelease CRD](../components/helm/helmreleases.md)
+- [Notification Controller](../components/notification/controller.md)
+    - [Provider CRD](../components/notification/provider.md)
+    - [Alert CRD](../components/notification/alert.md)
+    - [Receiver CRD](../components/notification/receiver.md)