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

Remove a previous workaround and use v0.7.1.

Original-Bug: 376
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/57180

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 9130758cd71e4cf..260b0cb56ee01da
Roller-URL: https://ci.chromium.org/b/8838726558488790320
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8838726558488790320
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ia70c35e8bb82a0971dc5c268842106e1f52cf26f
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/57321
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 9130758..260b0cb 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 9130758cd71e4cf081395835cfbf685bc0bf069c
+Subproject commit 260b0cb56ee01da8d6676db489dc3b25780e9c70