[v3-future] Show navigation sidebar in documentation

[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.

[dlqqq]

Setting this to P1 on our project board because we have received so many issues due to gaps in our documentation. It is absolutely critical to ensure that Jupyter AI v3 is more well-documented. Lowering the number of new bug reports will greatly improve the maintainability of Jupyter AI in the long term.