docs: add Python gRPC client examples

[beanscg]

Summary

• add Python examples for calling the public Entity gRPC service
• show request construction for CreateEntity and AuthenticateEntity using generated stubs
• note that the example channel is for local development and TLS deployments should use grpc.secure_channel()

Fixes #100

Verification

• git diff --check
• /tmp/bean-gh472-venv/bin/python -m grpc_tools.protoc -I protos/v1 --python_out=/tmp/bean-gh472-grpc-out --pyi_out=/tmp/bean-gh472-grpc-out --grpc_python_out=/tmp/bean-gh472-grpc-out protos/v1/vault.proto
• /tmp/bean-gh472-venv/bin/python -m py_compile /tmp/bean-gh472-grpc-out/vault_pb2.py /tmp/bean-gh472-grpc-out/vault_pb2_grpc.py
• generated module import and request construction check