[third_party/pigweed] Roll 2 commits

90d97fa3b4292c3 docs: Remove symlinks to files that were removed
3f2ce7672ef5c6c docs: Mention upstream development guide in contri

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 22db84fc5868737..90d97fa3b4292c3
Roller-URL: https://ci.chromium.org/b/8766340092329822385
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Idf60a4eb5c7ab911b2887125e87cc8184ff90682
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/kudzu/+/177516
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 22db84f..90d97fa 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 22db84fc5868737c55afeb1302bc1f0677b5b18e
+Subproject commit 90d97fa3b4292c39074e6f2b94dbd1cffa6ee318