[ADD] : new blog demo snippets

- Task: https://www.odoo.com/web?debug=#id=9702&view_type=form&model=project.task
- Pad: https://pad.odoo.com/p/r.yj9QgkBSrQnoIhel
Please put it in a separate file (snippets.xml). This way we avoid mixing regular templates and snippets.