docs: EXPOSED-672 Update the DAO's Relationships topic and add a new example project

[vnikolova]

Description

Updated and extended the "Relationships" topic under "Deep dive into DAO" and created a new Gradle project for the examples.

1. Examples

• Move all code examples to a new project exposed-dao-relationships.
• Update the example for a parent-child reference to build upon the StarWarsFilmsTable.

2. Content

• Add a topic intro, as well as short intros to each relationship type.
• Split content into sub-sections where relevant.

To do:

• Add API links

---

Type of Change

• Documentation update

Checklist

• The build is green (including the Detekt check)
• Documentation for my change is up to date

---

Related Issues

EXPOSED-672

[bog-walk]

Please check comments.
The only big one is the use of backReferencedOn() in UserRatingEntity.