Set up infrastructure for Jest (JavaScript) tests

[Eric-Arellano]

Prework for #208.

Abby and I decided that we want Jest tests for two purposes:

1. Checking that web components are in sync between the Pytorch and Furo themes, e.g. we didn't forget to update one of the folders.
   1. We could do this either by using Snapshot testing or by simply checking that the file contents are equal.
2. We have some non-trivial logic in Fix previous releases sidebar feature for ecosystem projects #267 and want to write automated unit tests.

This PR adds the basic infrastructure, like setting up CI to install Node.js. That will make it easier to review follow-up PRs that add actual tests.

We could do this either by using snapshot testing

I think we'd benefit from adding snapshot testing for the top nav bar rendering correctly. I want to do this in a follow up PR so we can properly decide if we want to add snapshots or not; the decision to use snapshots is separate from whether to use Jest generally.

[javabster]

LGTM 🚀