[roll third_party/pigweed] pw_web: Add user guide page for features and filter syntax

Original-Bug: 307560371
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/203632

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 9a0e93fe506c9f5..b21f1b705dbf644
Roller-URL: https://ci.chromium.org/b/8749912889764728465
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ie2465f00163d9bb4241df84c14ef87d8e2a70dc9
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/examples/+/205794
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 9a0e93f..b21f1b7 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 9a0e93fe506c9f594fdb4927a5e663c4465e12e2
+Subproject commit b21f1b705dbf644d0d97817705a3231304f21fbc