This PR adds a new Reference top-level section (folder is called ref as reference is already taken by the Reference Gallery) that for now includes this:

• Plot Options
  • Data options

The Plot Options page includes a table per options group (e.g. Data Options, Resampling Options). These tables are automatically built from the class docstring of the HoloViewsConverter object. This is done via a custom Sphinx extension that leverages numpydoc to parse the docstring. To get the table to format the content nicely, I had to touch the docstrings in a few places. Ultimately, we should enable some sort of docstring validation to make sure it is appropriately formatted.

The Data Options page is mostly a placeholder at the moment (for future PRs). While the Plot Options page only contains tables, this page should contain the table but also subsections that give more information about the options and most importantly include plots demonstrating their effects. This is why this page is a Notebook.

When this PR is merged, we should quickly be able to remove the Customization user guide, redirecting to the Plot Options page.

EDIT: