Add MDN issue to PR template

[domenic]

https://github.com/whatwg/spec-factory/blob/main/PULL_REQUEST_TEMPLATE.md.template should be updated with a new checklist item like:

- [ ] [MDN Issue](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) is filed (if applicable): …

Then we need to update https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests to give the right instructions for documentation.

@sideshowbarker maybe you could work on the whatwg/meta changes first?

[annevk]

@sideshowbarker did this drop of your radar?

[sideshowbarker]

@sideshowbarker did this drop of your radar?

Apologies for having neglected it — just now opened whatwg/meta#226

[sideshowbarker]

Apologies for having neglected it — just now opened whatwg/meta#226

Superseded by whatwg/meta#227

[annevk]

I merged that, do you want to PR spec-factory? I can probably review today and merge and deploy tomorrow (generates 17 - 1 PRs and I'm not sure I want to deal with that today).