Use the Redis cube mark (PNG) for the catalog icon

Reverting the last commit's wordmark SVG: the catalog grid is
square-card-shaped and the wordmark rendered too small / off-balance
against peer cards (mongodb, pinecone, qdrant). Switch back to the
460x460 cube mark from the Redis org GitHub avatar, which is square
and matches the visual rhythm of the catalog.

- Restore `docs/integrations/assets/redis.png` (cube on red, 460x460).
- Drop `docs/integrations/assets/redis.svg`.
- Update `catalog_icon` back to the PNG.

Verified with `mkdocs build --strict`.

Author: nkanu17

docs/integrations/assets/redis.png

@@ -1,7 +1,7 @@
 ---
 catalog_title: Redis
 catalog_description: Vector, hybrid, and SQL search plus session, memory, and semantic cache for agents
-catalog_icon: /integrations/assets/redis.svg
+catalog_icon: /integrations/assets/redis.png
 catalog_tags: ["data","mcp"]
 ---