Feat/spec reusability : Initial PoC

This MR introduces a re-organization in spec generation:

spec-assets folder (new). It contains:

• schema-src folder with the authored schemas, the ones we need to edit.
• schema-definitions folder with common definitions that apply to all schemas

The script generate-schemas.js generates the delivered schemas that remain in the public folder. (Nothing changes from current approach)

This reorganization transitions us towards a more maintainable specifications without losing the initial approach i.e auto contained assets.

This in the future could grow with automatic generation of the LD Context, etc.