Better document where PRs/Issues are sent/discussed

While the project uses GitHub workflows for PRs and issues, it uses the dev@ list to discuss questions. Should either move notifications to dev@ to surface them where people are actively looking, or should better document that all this traffic is on issues@ and ensure more developers are subscribed there.

Discovered wondering why my doc PR was never merged (I'm hoping to contribute/organize more documentation going forward, since I'm using ponymail more now).
https://github.com/apache/incubator-ponymail-site/pull/4

Start by reviewing the project's GitHub issue and pull request workflow, the dev@ and issues@ mailing-list references, and the linked documentation PR #4. Determine whether the project has chosen notification destination and subscription guidance; done means documenting that decision clearly or recording the agreed notification change.

Written by the indexing model from the issue text.