[DOC] Provide an intro to cuSignal in the API docs site as good as, or better than, what is given in the README

[taureandyernv]

Report needed documentation

Report needed documentation
In the RAPIDS docs site, https://docs.rapids.ai/api/cusignal/stable, there needs to be some introduction to cusignal and a quick guide, similar to other repos and what cusignal has in its README, https://github.com/rapidsai/cusignal#readme,

Describe the documentation you'd like

• Intro to cusignal
• Guide to using cusignal

Steps taken to search for needed documentation
List any steps you have taken:

• Looked in the Docs site
• looked at the readme