New explainer for Private Exclusion Targeting

Hi Web Advertising BG,

We've been working on the idea of "exclusion targeting" in a privacy focused way, and have shared a new explainer<https://github.com/w3c/web-advertising/blob/master/PETREL.md>, Private Exclusion Targeting Rendered Exclusively Locally ("PETREL") in the web-adv GitHub.

While we call it out in the explainer itself, I'd also want to emphasize that this idea had a ton of overlap with TURTLEDOVE. We were very happy that we were able to leverage and build from those ideas.

We'd love to present and get feedback on it on tomorrow's call, if possible..

Cheers,
Erik