Add documentation for user login and registration use cases and tests.

[alfred-mikhael]

Description

Add documentation for user login and registration use cases.

Design Patterns

None

SOLID Principles

None

Breaking Change

• [ ] Check this if your changes will cause merge conflicts in other branches.

Checklist:

• [x] My code follows Java style conventions
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have added Javadocs for new classes and methods
• [x] My changes generate no new relevant warnings
• [x] I have added tests for any new functionality added
• [x] New and existing unit tests pass locally with my changes