[roll third_party/pigweed] docs: Move CoC to Contributors section of sitenav

We are attempting to make the sitenav less of a chaotic, sprawling
mess. The text of the Code of Conduct (CoC) suggests that the code
primarily applies to upstream Pigweed contributors. Therefore,
putting this doc under the Contributors section of the sitenav
seems reasonable.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/177071

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 16d9aa7b128ef89..a1f371c42bc7e59
Roller-URL: https://ci.chromium.org/b/8766761592342596417
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I3f644e8684d1afbb4738cf87fbeaf42bc6929026
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/176986
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 16d9aa7..a1f371c 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 16d9aa7b128ef89ce3fa57f03c4591b54c5324d9
+Subproject commit a1f371c42bc7e593304e5a060ee9475d62396b5a