[roll third_party/pigweed] docs: Upgrade sphinxcontrib-mermaid

- Now using mermaid v10.2
- Set mermaid svg output to always have a white background.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/149850

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: fecb6d78396ab31..6a6ee80152a0cc2
Roller-URL: https://ci.chromium.org/b/8779539892951060897
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I17ae82c09f34a978c2fb18e9812bb68709379c9b
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/149820
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index fecb6d7..6a6ee80 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit fecb6d78396ab316b430ae2fc650ef63599054da
+Subproject commit 6a6ee80152a0cc25b076cdd1873524adb40dc975