Documentation reorganization

[tupui]

We have 3 documentations–and some notebooks around:

• https://rbc.readthedocs.io/en/latest/?badge=latest
• https://xnd-project.gitbook.io/rbc/
• OmniSci gitbook

I suggest we merge the resources into a single document. Doing so, I also propose to move to the pydata-sphinx-theme used by the pydata/scientific python community. This would modernise the look and feel and be more inviting for newcomers coming/used to these external projects.