Refactor OWNERS files in platform/build/soong.

Pulls out all of the per-file rules into their relevant directories
so platform/build/soong:/OWNERS can be included everywhere we need
the list of build system owners.

Test: treehugger
Bug: 170407947
Change-Id: I5a4b2d252bd9b3c280cd9954cf0e65ac21153ede
diff --git a/docs/OWNERS b/docs/OWNERS
new file mode 100644
index 0000000..d143317
--- /dev/null
+++ b/docs/OWNERS
@@ -0,0 +1 @@
+per-file map_files.md = danalbert@google.com, enh@google.com, jiyong@google.com