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.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

frontend/src/index.css

@@ -72,7 +72,7 @@
  *
  * `--primary` is #4a7189, not the identity blue #59849b: the latter measures
  * 4.04:1 against white, below AA for the label sitting on a bg-primary fill.
- * #59849b remains the wordmark colour (org/design/fluksio-*.svg). See the root
+ * #59849b remains the wordmark colour (index/src/assets/fluksio-*.svg). See the root
  * DESIGN-GUIDELINES.md → Colour tokens.
  */
 :root {