📝(contributing) update contributing policy (#1192)

## Purpose
Update the issue template to help contributors

Please ensure the following items are checked before submitting your
pull request:
- [x] I have read and followed the [contributing
guidelines](https://github.com/suitenumerique/docs/blob/main/CONTRIBUTING.md)
- [x] I have read and agreed to the [Code of
Conduct](https://github.com/suitenumerique/docs/blob/main/CODE_OF_CONDUCT.md)
- [x] I have signed off my commits with `git commit --signoff` (DCO
compliance)
- [x] I have signed my commits with my SSH or GPG key (`git commit -S`)
- [x] My commit messages follow the required format: `<gitmoji>(type)
title description`
- [ ] I have added a changelog entry under `## [Unreleased]` section (if
noticeable change)
- [ ] I have added corresponding tests for new features or bug fixes (if
applicable)

---------

Signed-off-by: virgile-dev <virgile.deville@beta.gouv.fr>

.github/ISSUE_TEMPLATE/Bug_report.md

@@ -6,6 +6,10 @@ labels: ["bug", "triage"]
 
 ## Bug Report
 
+**Before you file your issue**
+- Check the other [issues](https://github.com/suitenumerique/docs/issues) before filing your own
+- If your report is related to the ([BlockNote](https://github.com/TypeCellOS/BlockNote)) text editor, [file it on their repo](https://github.com/TypeCellOS/BlockNote/issues). If you're not sure whether your issue is with BlockNote or Docs, file it on our repo: if we support it, we'll backport it upstream ourselves 😊, otherwise we'll ask you to do so.
+
 **Problematic behavior**
 A clear and concise description of the behavior.