[roll third_party/pigweed] docs: Refactor the getting started section

There should be a getting started homepage that lists all the
various paths for getting started with Pigweed. Also, we're
preparing for a Bazel-focused getting started path.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/176331

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 40dce40e35ae626..1a9f0f5e7277a1b
Roller-URL: https://ci.chromium.org/b/8766965343327754033
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ic57a3518b542d6a7130c5aa8384e9a573cecb04e
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/176613
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 40dce40..1a9f0f5 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 40dce40e35ae6269c8a1177600ec2d2315836499
+Subproject commit 1a9f0f5e7277a1b5b0b7cbe60fed0386413957f6