roll: third_party/pigweed 57a886d..53ccf2c (2 commits)

53ccf2c:https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/502625 pw_rpc2: Integrate Doxygen comments; minor docs tweaks
03a00c7:https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/503126 pw_rpc2: Add docs landing page

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: 57a886dc82d457..53ccf2c2b50266
Roll-Count: 1
Roller-URL: https://cr-buildbucket.appspot.com/build/8667862243892138545
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I4af019d6310b3637f6a83398c7c782734cb4198b
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/examples/+/503905
diff --git a/third_party/pigweed b/third_party/pigweed
index 57a886d..53ccf2c 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 57a886dc82d457b9530e1f8f7d236d46b8cce940
+Subproject commit 53ccf2c2b50266dc9a9968dc1760d0c270afeed3