docs: update stackable overview image (#466)

* docs: update stackable overview image

* add old overview image to retired_files to delete it

* Made corners on stackable overview image transparent.

* Fixes to bottom part of image to attempt and remove waviness.

---------

Co-authored-by: Sönke Liebau <soenke.liebau@stackable.tech>

config/retired_files.yaml

@@ -3,4 +3,4 @@
 # This is uncommented as I had issues with everything being deleted when this was just present as an empty key.
 # May be something to investigate.
 retired_files:
-  - scripts/run_test.sh
+  - .readme/static/sdp_overview.png

template/.readme/partials/borrowed/footer.md.j2.j2

@@ -3,7 +3,7 @@
 
 This operator is written and maintained by [Stackable](https://stackable.tech) and it is part of a larger data platform.
 
-![Stackable Data Platform Overview](./.readme/static/borrowed/sdp_overview.png)
+![Stackable Data Platform Overview](./.readme/static/borrowed/stackable_overview.png)
 
 Stackable makes it easy to operate data applications in any Kubernetes cluster.