[roll third_party/pigweed] docs: Update Sphinx; remove Markdown; add CSS

This completes the update of Sphinx and RST by removing the legacy
Markdown support. It also adds custom CSS skeleton.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/40621

third_party/pigweed Rolled-Commits: 861dbdbd66f0dc0..b19140275a0ca1b
Roller-URL: https://ci.chromium.org/b/8848238033116965744
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8848238033116965744
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ia693da28e85ac77c6a2e7f296d39be6c659118ac
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/43480
Reviewed-by: 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 861dbdb..b191402 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 861dbdbd66f0dc0a96c413fb00d65c5c30082101
+Subproject commit b19140275a0ca1bf62be7acea757d794871f6e43