BACKEND - Seeder for Exercise data

## What
Create a seeder that populates database with Exercises (and Units, Modules)

## Why
To get realistic data for the development process.

## How
- Seeder for Exercise data
- Seeder for Unit data
- Seeder for Module data
- Add to data context
- Verify data in database

Data entries should be inspired by Figma example as they are realistic.