[roll third_party/pigweed] docs: Rework first-time setup

Reworks Pigweed's getting started documentation around first-time system
setup, dividing the guides into OS-specific pages.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/185730

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 3e12ae03b72ab8e..a49938f32d4beff
Roller-URL: https://ci.chromium.org/b/8759986912181805745
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I6f99436a62673edb42005ad401d89a1e42beb284
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/186477
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 3e12ae0..a49938f 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 3e12ae03b72ab8eade72e62d4960152593440856
+Subproject commit a49938f32d4beff6677ca16bfdce9f0aa8e0fc6b