Re-organize database documentation and fix some parts of the docs

[LoicPoullain]

Issue

The database section needed reorganization. The documentation now highlights that it is possible to use another ORM than TypeORM.

Solution and steps

• [x] Re-organize database docs
• [x] Fix minor issues

Checklist

• [x] Add/update/check docs (code comments and docs/ folder).
• [x] Add/update/check tests.
• [x] Update/check the cli generators.