[roll third_party/pigweed] docs: Add protobuf style guide

Introduces a style guide for proto files throughout Pigweed.

Original-Bug: 232867615
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/190723

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 64bfe3952506478..dc5ef2e7790bfa5
Roller-URL: https://ci.chromium.org/b/8754196057196367601
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: If8487f3ad63815746b0c92e9cc81c3a466acf4f0
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/examples/+/195093
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 64bfe39..dc5ef2e 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 64bfe39525064789c2f79efa9f2d9afe06a0af0c
+Subproject commit dc5ef2e7790bfa526901e3de3ef41f43b48fb823