Renewing perception metrics documentation

The current documentation is split between the human-readable docs at https://jderobot.github.io/PerceptionMetrics/ and the docstring-driven API docs at https://jderobot.github.io/PerceptionMetrics/py_docs/build/html/index.html.

Combining these into a clearer documentation structure would make it easier for users to find guides for datasets, models, CLI usage, metrics, and data formats, while still keeping the detailed Python API reference available.

Reference style:
https://carla.readthedocs.io/en/latest/python_api/