Add more / finer-grained OWNERS

This will help with review load and finding an owner once mandatory
owner review is enabled.

Change-Id: I1a90b83fed68f062c1406552aec218bf9c2092ce
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/67821
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
diff --git a/docs/OWNERS b/docs/OWNERS
new file mode 100644
index 0000000..72e8ffc
--- /dev/null
+++ b/docs/OWNERS
@@ -0,0 +1 @@
+*