Improve diagrams for the Document Service concept introduction

This PR replaces the static, Miro-based diagrams for the Document (Service API) conceptual introduction by new diagrams generated by Mermaid.js.

It will make diagrams easier to read & maintain.

Mermaid styles have been adapted to be closer to the original Miro diagrams. All diagrams are responsive and will fit to screen size as much as possible since they benefit from the zooming mixins recently created for the backend customization introduction (see commit 90cc1f2).

Direct preview link 👉 here

strapi / documentation

Improve diagrams for the Document Service concept introduction #2187