BQ API references could be more user friendly

[mmahsereci]

Is your feature request related to a problem? Please describe. API reference structure of probnum.quad could be more user friendly.

Describe the solution you'd like.

Currently everything is top level. It would be cleaner with some hierarchy. Example:

• quad
  • bayesquad
  • bayesquad_from
  • solver
    • stopping_criteria
    • belief_updates
    • ...

Additional context

• Similar adjustments are made in the linalg package. It would be great if the API structures were aligned.
• Some structural changes may need to be made in the quad package before this issue can be addressed. See list at the end of #420 and discussion in #570