[roll third_party/pigweed] docs: Split the style guide: commit style

Initial CL in a series.

- Set up ToC for the new pages under Contributing > Style Guide > ...
- Move the commit message style guide into a separate page
- Rework the commit message style guide with headings and more content

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/178490

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 9335766691d9ddd..af237dbf10c3843
Roller-URL: https://ci.chromium.org/b/8765493914134833873
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ie6d3c77c48ef1091e1cac4f6c5bfe85ff2168315
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/178863
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 9335766..af237db 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 9335766691d9ddd8c44cd9116889af11506bbb9e
+Subproject commit af237dbf10c3843a006c789a7c40ea250dab68fa