Add API reference - Githubissues

This PR adds an API reference generated from the code. It comprises the user-faced part of plinder.core and plinder.core.scores.

I would need some help for the following parts:

I think plinder.core and plinder.core.scores should have a module docstring in __init__.py that explains what the respective package is about.
Similarly a short overview on the entire user-faced API would be helpful in docs/api/index.md
Currently the docstrings of the public functions/classes misses some section in the docstring (e.g. PlinderSystem misses the Attribute section).
For the functions that have a complete docstring, the descriptions are often quite generic, i.e. the docstring does not convey much more information than the function signature itself. I think more details would be helpful to new users (including me :wink:).
Optionally doctests would be helpful in the docstrings, as the give users a graspable example how the functionality can be used.
Note that numpydoc requires docstrings to be written in reStructuredText instead of Markdown. They are very similar but there are small nuances where they are different. For example monospace is formatted with double backticks instead of single ones.

plinder-org / plinder