[WM] flatten the mentor role

We decided to drop the mentor / mentee distinction in our retro and just
make it a mutual mentoring role.
I wrote the first version (the "Advocate for the mentee within the company" version), and essentially what I was thinking about was making your pair's accomplishments known by people outside of the dev team and recommending them for responsibilities that they could take on (like, "Oh? You're thinking about SEO? You should talk to Sally, she's done SEO work before.").