[IMP] digest: make tips bioutifoul, and add moar tips

PURPOSE

Make digest email and tips more appealing. The goals of these tips are

  * to encourage the adoption of other apps (Did you know ?);
  * to make Odoo look more fun (Fun tips and tricks, young and dynamic style);
  * to show social proof and increase trust (emphasis on already existing
    projects / customers to);

SPECIFICATIONS

Improve existing tips to be more inlined with new digest styling.

Add new tips, notably

  * Invite team members;
  * Learn about Odoo (eLearning platform);
  * allow push notifications;

LINKS

Task ID 2197417
PR odoo/odoo#51619

Co-Authored-By: Elisabeth Dickinson <edi@odoo.com>
Co-Authored-By: Thibault Delavallée <tde@odoo.com>