I think github might automatically use the rel="me" HTML option if you add a link to your Mastodon profile? If so, you don't need to add any special HTML.
rel="me" was supposed to be the standard way to verify your account on other platforms, and if it had happened that way we wouldn't need to insert special bits of HTML on websites. Unfortunately Facebook and Twitter decided not to use it, perhaps they didn't like people being on other platforms?