[roll third_party/pigweed] gitignore: Ignore generated doc previews

Some .rst preview tools put files in docs/_build by default.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/53441

third_party/pigweed Rolled-Commits: ce5eaf4017150cd..ef336f0a82855f5
Roller-URL: https://ci.chromium.org/b/8841529065262228272
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8841529065262228272
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ibb5a837486fd1cdd6582ee891b5bc3a328307643
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/53397
Reviewed-by: 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 ce5eaf4..ef336f0 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit ce5eaf4017150cd4afdf90f2fd51651f1eb9a02b
+Subproject commit ef336f0a82855f5f65b4b475f9af8560f51fcf51