[TASK] Create testing guidelines documentation

## Description
This task will create documentation on Cypress testing best practices, for example element selection strategies.

- Will serve as a reference for all team members when writing new tests and will help maintain consistency across the codebase.

## Acceptance Criteria

- [ ] Documentation has been created (e.g., in `docs/testing.md` or similar location)
- [ ] References Cypress official best practices
- [ ] Docs reviewed / approved by the team

Start by reviewing the repository's documentation structure and Cypress official best practices, especially element selection strategies. Create the guidance in docs/testing.md or the appropriate documentation location, covering consistent Cypress testing practices. Done means the document exists, links to Cypress guidance, and has been reviewed and approved by the team.

Written by the indexing model from the issue text.