Change all Docstrings to Markdown instead of reST.

[CSchoel]

See #75. Staying with reST makes sense as long as we use Sphinx, but even that can use the MyST parser. We will most likely want to switch since for nolds it is also important that developers can read the docstring straight from the code (see considerations in #66).

• [ ] Review which features from reST we need.
• [ ] Translate one complex docstring from reST to Markdown to estimate difficulty of changing styles.
• [ ] (optional) Translate the rest of the docstrings.