[roll third_party/pigweed] pw_protobuf: Squash docs into a single file

Squashes the 3 separate docs for pw_protobuf into a single docs.rst
to improve discoverability.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/58200

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: e93ccb13002325a..f1672fbfda39e5f
Roller-URL: https://ci.chromium.org/b/8837910575418858385
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8837910575418858385
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I2e4cea90b7c4547c4c371ee0f9d8faccd194b9c0
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/58240
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 e93ccb1..f1672fb 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit e93ccb13002325a78690435fdd19a685d4b00e64
+Subproject commit f1672fbfda39e5fd658296baa489544ddfddbe5e