_content/blog/llmpowered: set background for the diagram

Adding the white background improves displaying the RAG server diagram
on the dark theme.

Fixes golang/go#69517

Change-Id: Icbe6248280814a2890667bd6a226722226ad9a74
Reviewed-on: https://go-review.googlesource.com/c/website/+/613435
Reviewed-by: Hyang-Ah Hana Kim <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Reviewed-by: Eli Bendersky <[email protected]>
TryBot-Bypass: Eli Bendersky <[email protected]>