Fix how pw_build docs are used.

Change-Id: Ic6d357a116f0da8b167a1b9618ca84f6513c5ec5
diff --git a/docs/BUILD.gn b/docs/BUILD.gn
index ac10d91..79aa782 100644
--- a/docs/BUILD.gn
+++ b/docs/BUILD.gn
@@ -31,9 +31,10 @@
   deps = [
     ":core_docs",
     "$dir_pw_bloat:docs",
+    "$dir_pw_build:docs",
     "$dir_pw_docgen:docs",
-    "$dir_pw_dumb_io_baremetal_stm32f429:docs",
     "$dir_pw_dumb_io:docs",
+    "$dir_pw_dumb_io_baremetal_stm32f429:docs",
     "$dir_pw_dumb_io_stdio:docs",
     "$dir_pw_module:docs",
     "$dir_pw_preprocessor:docs",
diff --git a/pw_build/BUILD.gn b/pw_build/BUILD.gn
index 3316607..aaa8e2d 100644
--- a/pw_build/BUILD.gn
+++ b/pw_build/BUILD.gn
@@ -12,6 +12,8 @@
 # License for the specific language governing permissions and limitations under
 # the License.
 
+import("$dir_pw_docgen/docs.gni")
+
 # Standard compiler flags to reduce output binary size.
 config("reduced_size") {
   cflags = [
@@ -49,3 +51,9 @@
 config("pw_default_cpp") {
   configs = [ ":cpp17" ]
 }
+
+pw_doc_group("docs") {
+  sources = [
+    "docs.rst",
+  ]
+}
diff --git a/pw_build/build.rst b/pw_build/docs.rst
similarity index 100%
rename from pw_build/build.rst
rename to pw_build/docs.rst