roll: third_party/pigweed pw_async2: Expand SKILL with guidance on designing async flows

Adds a new reference doc to the skill which explains how to approach
designing async code by first drafting the system as it would look
using coroutines, then transforming that into a series of composite
futures managing relevant state.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/469505
Original-Revision: b484558db0228164e6055f92bef64b5008527a7a

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: 98a57261d86c84..b484558db02281
Roll-Count: 1
Roller-URL: https://cr-buildbucket.appspot.com/build/8671569067550525361
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I386629e86d9f5d108d542a94c591f69cb09371cd
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/examples/+/470325
diff --git a/third_party/pigweed b/third_party/pigweed
index 98a5726..b484558 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 98a57261d86c8413b9b32bea70f6cb18f4309aae
+Subproject commit b484558db0228164e6055f92bef64b5008527a7a