Create notebook PR template

[martinacantaro]

Require PRs to follow the Style Guide

Require a statement detailing what the notebook adds in the context of our documentation and why it’s important.

Require checking that the content isn’t already covered by another notebook

[OriolAbril]

The pull request template for pymc-examples is currently like this https://raw.githubusercontent.com/pymc-devs/pymc-examples/main/.github/PULL_REQUEST_TEMPLATE.md, the contributing guide is https://github.com/pymc-devs/pymc-examples/blob/main/CONTRIBUTING.md

[OriolAbril]

Require a statement detailing what the notebook adds in the context of our documentation and why it’s important.

I would try to continue enforcing the contributing guide, and have PRs link to an issue (which could be about a missing notebook), otherwise it can be frustrating for contributors to start a notebook and submit a PR after significant work only to have it rejected