[roll third_party/pigweed] docs: More readable text selection colors

- Add `::selection` css rules for more readable colors.
- Use normal mixed case text for h4, h5, h6 headings.
- Address unresolved comments from
https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/57064

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/57324

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: d1ca56c8cd410cd..1a88725edb3e6ef
Roller-URL: https://ci.chromium.org/b/8838714748955502544
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8838714748955502544
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I52fc0b9f746e3f45d259b825873cd7cbe7610ba8
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/57333
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 d1ca56c..1a88725 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit d1ca56c8cd410cdf496d22470fca461ad71dfd34
+Subproject commit 1a88725edb3e6ef4c0375cc10d897ae00885312c