[roll third_party/pigweed] docs: Update style_guide with admonitions

- Removed blockquotes around bulleted lists
- Fixed indentation
- Removed extra lines before+after headings
- Added headings to where appropriate
- Created the ':class: checkmark' admonition, example:

.. admonition:: **Yes**: Do this
   :class: checkmark

   Content here.

.. admonition:: **No**: Avoid this
   :class: error

   Content here.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/73220

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 11449fbe4fc12bc..6a1484b27828017
Roller-URL: https://ci.chromium.org/b/8828289096822913905
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8828289096822913905
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ia2548db3f2b20efa7c02f0d95595311efbc14177
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/73182
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 11449fb..6a1484b 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 11449fbe4fc12bc4c04da26e0a0ba3e75871cacf
+Subproject commit 6a1484b27828017bec41e8a9551e34d80c26489d