Provide access to the documentation for older version

[fcollonval]

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

[jtpio]

Yes we should add a way to switch between versions.

Here is an example URL for Notebook 6: https://jupyter-notebook.readthedocs.io/en/v6.5.4/