Rename the workspace layout: fluksio -> app, website -> index

The repo moves to git.stroblme.de:Fluksio/app.git and its sibling to
Fluksio/index.git, so every cross-repo path reference moves with it. The
architecture sources the roadmap cites now live in the docs repo under
architecture/, and the brand marks in the index repo's src/assets.

Also replaces the placeholder README with one that reflects the actual layout.

package.json

@@ -1,5 +1,5 @@
 {
-  "name": "fluksio",
+  "name": "fluksio-app",
   "private": true,
   "workspaces": [
     "frontend"