Provide access to the documentation for older version

## Description

I cannot figure out how to access the documentation for previous major version of notebook

## Reproduce

1. Go to https://jupyter-notebook.readthedocs.io/
2. There is not version selector to jump to older documentation version

I tried to set an arbitrary version in the URL but it seems the documentation is not published (or I don't use the right version pattern).

## Expected behavior

We can access older version of the documentation.

> In JupyterLab the documentation is available for any minor versions for supported major versions. And for the latest version of unsupported major versions.
> The configuration of a version switcher can be found there: https://github.com/jupyterlab/jupyterlab/blob/304377719bbd3d7c53000faa341f020110aac815/docs/source/conf.py#L280

Start with the JupyterLab version-switcher example in docs/source/conf.py linked in the issue, then inspect how the Jupyter Notebook documentation is currently published at readthedocs.io. Done means supported older major-version documentation is reachable through a visible version selector.

Written by the indexing model from the issue text.