docs(back-end): add schema

[JowiAoun]

🆘 What is the problem at hand?

We recently introduced a database for portal, using DrizzleORM. It is time to write the initial schemas previously discussed.

🚀 What is the impact of the problem? How is this problem important? How does it affect users/developpers?

Beginning with writing the database schemas allows us to have a better overview of requirements and lets makes other developers aware with the type of data they are working with.

💀 Additional context

We have discussed the initial database schemas previously in issue #71, and are now implementing them with DrizzleORM.

[HasithDeAlwis]

@JeremyFriesenGitHub @JowiAoun Priority to get this merged in ‼️‼️

[JeremyFriesenGitHub]

@JeremyFriesenGitHub @JowiAoun Priority to get this merged in ‼️‼️

It's ready for review @HasithDeAlwis