[roll third_party/pigweed] pw_crypto: Add initial documentation

Add initial documentation for a taste of what is coming up.

Test: `pw presubmit`

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/50320

third_party/pigweed Rolled-Commits: 8ec2cf86d56318b..6a23acfd434c27e
Roller-URL: https://ci.chromium.org/b/8841878161606985312
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8841878161606985312
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I06ccc4e00b5a427ac964e5908c073239899565be
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/52889
Reviewed-by: 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 8ec2cf8..6a23acf 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 8ec2cf86d56318b8fb068892ca4892953aa83acf
+Subproject commit 6a23acfd434c27ed121ff7032ff67a183961be85