[roll third_party/pigweed] docs: Restore blockdiag diagrams

- Replace simple blockdiag diagrams with Mermaid.
- Use the rendered SVG for other blockdiag diagrams for now.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/60060

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 16dcfa005c26c40..1d221243e6fadc8
Roller-URL: https://ci.chromium.org/b/8836638368393820497
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8836638368393820497
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: If6b9a87ec5e58d7c44fa57dbdecc101c150e600c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/60142
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 16dcfa0..1d22124 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 16dcfa005c26c40f1da520914c6a3ab447a70ea3
+Subproject commit 1d221243e6fadc86c1ee87f1444633af27611143