move "storage" section out of the extensions section since it is no longer a (meta) extension

docs/ocis/storage/spacesregistry.md

@@ -0,0 +1,20 @@
+---
+title: "Spaces Registry"
+date: 2018-05-02T00:00:00+00:00
+weight: 9
+geekdocRepo: https://github.com/owncloud/ocis
+geekdocEditPath: edit/master/docs/extensions/storage
+geekdocFilePath: spacesregistry.md
+---
+
+{{< hint warning >}}
+
+The current implementation in oCIS might not yet fully reflect this concept. Feel free to add links to ADRs, PRs and Issues in short warning boxes like this.
+
+{{< /hint >}}
+
+## Storage Space Registries
+
+A storage *spaces registry* manages the [*namespace*]({{< ref "./namespaces.md" >}}) for a *user*: it is used by *clients* to look up storage spaces a user has access to, the `/dav/spaces` endpoint to access it via WabDAV, and where the client should mount it in the users personal namespace.
+
+{{< figure src="/ocis/storage/static/spacesregistry.drawio.svg" >}}