Implement automatic documentation generation using, e.g., Sphinx

[matteoettam09]

It makes sense to have automatic docs generation. Use the design.md to inform docs of functions and be deprecated, as currently docs are either in the alkahest docs or in the codebase.

Together with this, a less-code API-based documentation shall be generated using gitbook, see as a nice example:
https://docs.zama.ai/concrete-ml
https://github.com/zama-ai/concrete-ml/tree/main/docs

Section https://docs.zama.ai/concrete-ml/references/api
is populated using https://github.com/ml-tooling/lazydocs
Sklearn docs is based on sphinx.