Support RTD version selector

[manics]

Proposed change

Older versions of the JupyterHub docs e.g. 3.0.0 had a version selector, allowing you to switch to other versions of the docs.

This seems to have been lost in JupyterHub 3.1.0, probably with the switch to the pydata theme?

Alternative options

Do nothing

Who would use this feature?

Users wanting to view docs for versions of JupyterHub other than the latest

(Optional): Suggest a solution

Instructions in pydata for enabling the version switcher:
https://pydata-sphinx-theme.readthedocs.io/en/stable/user_guide/version-dropdown.html

The main requirement seems to be a JSON file with a fixed URL containing the list of versions. Is this file automatically created by readthedocs, and can we automatically set this URL?

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[choldgraf]

The pydata theme should support RTD dropdowns out of the box...that is weird! Do you see any errors in the JavaScript console?

[manics]

There are no javascript errors.

I've found the original pydata-sphinx-theme PR: pydata/pydata-sphinx-theme#436
It doesn't look like the version selector is automatic- the PR added support for the version dropdown (disabled by default) and also added the repo-specific config to enable the dropdown.

The list of versions is manually managed, it's not automatically detected:
https://github.com/pydata/pydata-sphinx-theme/blob/main/docs/_static/switcher.json

The pydata-sphinx-theme release process includes this as a step:

- [ ] Update our version switcher .json file with the new version

[choldgraf]

The JSON switcher is a way to manually have a version switcher if you don't want to use the RTD one (and if you manually want to maintain versions in a single JSON). So maybe there's a feature flag for the RTD one too?