doc: add doc for manual re-generation of cab-token-generator protobuf classes (#1886)

Author: zhumin8

cab-token-generator/gencode-guide.md

@@ -0,0 +1,17 @@
+Code under `cab-token-generator/java/com/google/auth/credentialaccessboundary/protobuf` are generated manually.
+To re-generate, follow steps below:
+
+Determine the protoc version to use for generate, this guide will use v33.2 as example.
+Steps to generate the java code using protoc 33.2.
+1. Download these files from https://github.com/protocolbuffers/protobuf/releases/tag/v33.2
+```
+src/google/protobuf/duration.proto
+src/google/protobuf/struct.proto
+src/google/protobuf/timestamp.proto
+```
+2. Create a workspace in g3, copy the above files in `google/protobuf` directory.
+3. Run the following command to generate java code:
+```sh
+# in google3 directory
+~/.local/bin/protoc --java_out ~/Downloads/java-output -I. cloud/identity/unifiedauth/proto/client_side_access_boundary.proto
+```