Clean up quad api

[mmahsereci]

In a Nutshell

This PR cleans up the quad api structure and the docs (see linked issue)

Detailed Description

n.a.

Related Issues

Closes #574

[codecov]

Codecov Report

Merging #739 (78c5280) into main (35f50e4) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #739      +/-   ##
==========================================
+ Coverage   90.46%   90.48%   +0.02%     
==========================================
  Files         198      199       +1     
  Lines        7498     7516      +18     
  Branches      974      974              
==========================================
+ Hits         6783     6801      +18     
  Misses        481      481              
  Partials      234      234              

Impacted Files	Coverage Δ
src/probnum/quad/__init__.py	100.00% <100.00%> (ø)
src/probnum/quad/_bayesquad.py	100.00% <100.00%> (ø)
src/probnum/quad/_utils.py	93.54% <100.00%> (+0.21%)	⬆️
src/probnum/quad/integration_measures/__init__.py	100.00% <100.00%> (ø)
...quad/integration_measures/_integration_measures.py	100.00% <100.00%> (ø)
src/probnum/quad/kernel_embeddings/__init__.py	100.00% <100.00%> (ø)
...c/probnum/quad/kernel_embeddings/_expquad_gauss.py	100.00% <100.00%> (ø)
...robnum/quad/kernel_embeddings/_expquad_lebesgue.py	100.00% <100.00%> (ø)
...robnum/quad/kernel_embeddings/_kernel_embedding.py	89.28% <100.00%> (ø)
...probnum/quad/kernel_embeddings/_matern_lebesgue.py	100.00% <100.00%> (ø)
... and 15 more

[JonathanWenger]

Looks good! Thanks for taking care of the module structure and docs!