[ADD] Occupancy percentage widget

[aledeleonez]

Second task of the technical assessment.

Changes made:

• Backend(DashboardView): Implemented logic to calculate the occupancy percentage (Active Bookings / Total Rooms) * 100.
• Frontend(dashboard.html): Added a new widget similar to the ones that existed. Used 'floatformat' to display the percentage (50.00%).
• Tests (tests.py): Implemented new tests using TestCase to prove the new functionality.