[roll third_party/pigweed] docs: Split the style guide: Doxygen & Sphinx

- Move Sphinx style guide into a separate file
- Move Doxygen style guide into a separate file
- Add cards to the top of the main style guide page

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/178912

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 4e5595180701fcc..5eb5b51fb17e731
Roller-URL: https://ci.chromium.org/b/8765414745825931809
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I57fa07d073f53d017af976b696411a7d6d9aa665
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/178868
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 4e55951..5eb5b51 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 4e5595180701fcccd97442b850a629dccc1b89bb
+Subproject commit 5eb5b51fb17e73154eeb2e8d6d686c5aa9a6968a