Overhaul core Tracker: review `tracker-core` package documentation

Parent issue: https://github.com/torrust/torrust-tracker/issues/1181

After the core tracker overhaul, the crate documentation must be updated accordingly.

Example of current docs:

![Image](https://github.com/user-attachments/assets/cdcb0d40-dd02-4d0f-9654-ee88a7c56922)