pw_bloat: Remove .rst from generated size reports

This change removes the .rst extension from the ReST size report files
generated by pw_bloat to prevent Sphinx from checking that they've been
added to a toctree. (Apparently including a file doesn't count as using
it.)

Change-Id: I4edc2db832bac0f370f563ac4f5c68692ad6f85e
diff --git a/pw_protobuf/decoding.rst b/pw_protobuf/decoding.rst
index 7c75a2e..d0dbea3 100644
--- a/pw_protobuf/decoding.rst
+++ b/pw_protobuf/decoding.rst
@@ -18,7 +18,7 @@
 decode methods and a decode callback for a proto message containing each of the
 protobuf field types.
 
-.. include:: size_report/decoder_full.rst
+.. include:: size_report/decoder_full
 
 
 Incremental size report
@@ -28,4 +28,4 @@
 fields to the decode callback to demonstrate the incremental cost of decoding
 fields in a message.
 
-.. include:: size_report/decoder_incremental.rst
+.. include:: size_report/decoder_incremental