improve documentation layout

[dlqqq]

Problem

We are approaching the point at which there is too much text in the user documentation to fit in a single page. Without a high-level index of a page, e.g. in a left sidebar, users will have a hard time discovering the documentation relevant to whatever use-case they're looking for. However, with the Sphinx theme we are using (pydata-sphinx-theme), it is definitely possible to have subpages within the user documentation, indexed by a left sidebar. Take a look here:

Proposed Solution

Figure out what configuration is necessary to achieve this, and split the chat and magics documentation into separate pages.