Docs

[almarklein]

Prerequisite for https://github.com/plotly/dash-docs/pull/1039

• Tweaked the docstrings in the VolumeSlicer to be more usable in markdown.
• Added a docsting at the top of the module that defines VolumeSlicer, to specify additional reference docs.
• Added a new docs module with a few simple utilities:
  • A function to get the reference docs as markdown. -> To be used in dash-docs.
  • A function to add the reference docs to the readme. This function is called when the file is executed as a script.
• Added tests for the above.
• General improvements to the docs.

[almarklein]

(I know what to do about the Windows CI error, but I'll wait with a fix until we figure out how to move forward)

[emmanuelle]

I approved a bit too fast (there's the Windows CI problem you mentioned), just wanted to say that the approach is sound I think.