chore: add ERD to documentation and update README

[BramSuurdje]

This pull request includes updates to the documentation and the addition of an ERD diagram for the project. The most important changes include adding sections for documentation and deployment instructions in the README.md file, and creating an ERD diagram in the documentation/erd.md file.

Documentation updates:

• README.md: Added sections for documentation and deployment instructions, including steps to deploy the project using Docker Compose.

ERD diagram addition:

• documentation/erd.md: Created an ERD diagram using Mermaid syntax to illustrate the database schema.