feat: initial unit test with Jest

[alexanderscheibler]

• Configures Jest for unit testing
• Add Github Action for Jest
• Add unit tests for the Header component
• Define ScreenSizes on file on the "data/" folder, for easy reuse across the tests.
• Refactor some components, moving their data to the "data/" folder. Examples: Nav links, Helper cards.
• Refactor Header, particularly for mobile. Changes:

1. Instead of horizontal stack (which would create an horizontal scroll or have items out of screen as the menu grows), have items in vertical stack)
   Before / After

2. Collapse the menu after an item is selected, so the user can see the full page / item

3. Have both mobile and desktop menu share the same array data for easy of update, instead of having the items hard coded twice.