Cleanup PR's without activities

[NeOMakinG]

A lot of PR's are waiting for author and are inactive since more than 30 days, we discussed about a way to clean this with a generic message.

https://github.com/search?o=desc&q=org%3APrestaShop+is%3Apr+label%3A%22waiting+for+author%22+is%3Aopen+updated%3A%3C%3D2020-04-01&s=updated&type=Issues

It concern PRs with waiting for author labels

What have to be done :

• [ ] Redact an article in order to inform the community
• [ ] Redact a generic message that we can use
• [ ] Review PRs without activies and close them if they are inactive using the message