Remove dependency on jQuery

[kwsp]

Problem

jQuery was removed from Sphinx in ~2022 with the release of Sphinx 6.0.0.

I came across this because of broken search functionality at https://pydicom.github.io/pydicom/dev/index.html. I realised the problem is that jQuery is somehow not loaded so searchindex.js isn't loaded. However, this simple need to load a script is trivial in vanilla JS nowadays, as Sphinx by default uses.

If the maintainers don't want to remove jQuery for other downstream projects, at least we can start by porting some of the simply tasks to vanilla JS as Sphinx had done, so when jQuery breaks, it doesn't break the site.