add rel attribute to prevent clickjacking
add rel attribute to prevent clickjacking

`noopener` and `noreferrer` both address clickjacking vulnerabilities on `target="_blank"`, though `noopener` isn't supported in some legacy browsers, while `noreferrer` is supported, and is ignored by more recent browsers. So adding both is currently the easiest way to support everything.

grpc/grpc.io-docsy,grpc/grpc.io-docsy,google/docsy,google/docsy