roll: third_party/pigweed docs: Normalize SEED filenames

Currently SEED filenames start with the SEED number but then
include arbitrary text, e.g. 0002-template.rst. Automation of
the SEED index will be easier if we normalize the filenames
to only include the number, e.g. 0002.rst. SEED URLs will also
be more predictable, e.g. pigweed.dev/seed/0002.html. This is
prereq work for the migration of the docgen system to Bazel;
it will be hard to migrate the SEED index auto-generation to
Bazel if we don't make this change.

Original-Bug: 318892911
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/248452
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Original-Revision: f4e18d1488c9d462a0d0da3342763a8864cb5200

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: 55f09ff7516de0..f4e18d1488c9d4
Roll-Count: 1
Roller-URL: https://cr-buildbucket.appspot.com/build/8730630371001400001
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I22baeeb0860be7cd9ef466ea168ee1acaf27401c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/examples/+/250396
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 55f09ff..f4e18d1 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 55f09ff7516de0be1f75bf8154a9a370690f67c5
+Subproject commit f4e18d1488c9d462a0d0da3342763a8864cb5200