pw_protobuf: Add basic size report for decoder

This change adds a simple size report for the pw::protobuf::Decoder
class with a simple decode callback.

Change-Id: I773cc31df43c5aab1db7044d77439cf8513e6b68
diff --git a/docs/BUILD.gn b/docs/BUILD.gn
index 050b326..dee8a15 100644
--- a/docs/BUILD.gn
+++ b/docs/BUILD.gn
@@ -52,6 +52,7 @@
     "$dir_pw_module:docs",
     "$dir_pw_preprocessor:docs",
     "$dir_pw_presubmit:docs",
+    "$dir_pw_protobuf:docs",
     "$dir_pw_protobuf_compiler:docs",
     "$dir_pw_span:docs",
     "$dir_pw_status:docs",