pw_rpc: Documentation update

- Add documentation for the pw_rpc Python package.
- Discuss using proto3 instead of proto2.
- Add "other_deps" to pw_doc_group for expressing arbitrary deps.

Change-Id: I3c8baf2a7986a7d06721d76278794201259815a7
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/33104
Reviewed-by: Keir Mierle <keir@google.com>
Commit-Queue: Wyatt Hepler <hepler@google.com>
diff --git a/pw_rpc/BUILD.gn b/pw_rpc/BUILD.gn
index bb22728..06ba545 100644
--- a/pw_rpc/BUILD.gn
+++ b/pw_rpc/BUILD.gn
@@ -146,7 +146,10 @@
     "pw_rpc_protos/echo.proto",
     "pw_rpc_protos/internal/packet.proto",
   ]
-  group_deps = [ "nanopb:docs" ]
+  group_deps = [
+    "nanopb:docs",
+    "py:docs",
+  ]
   report_deps = [ ":server_size" ]
 }