Add search functionality to cellxgene-census docs

[ayushnoori]

Description

Add search functionality to cellxgene-census documentation website at https://chanzuckerberg.github.io/cellxgene-census/index.html.

Context

See https://cziscience.slack.com/archives/C04LMG88VKJ/p1689261048038369.

[ivirshup]

Some updates after an initial investigation:

• Sphinx by default includes search entirely client side search. E.g. you start a new project and run make html, there will be a functioning search bar
• Currently this feature is disabled in the census docs. The search index is still built, but the search bar isn't excluded by using a blank "searchbox.html" template. If I remove that, a search bar is visible but still seems non-functional due to a missing jquery. Still not sure how that is being excluded or how I can get it back.

[ivirshup]

As an alternative to client side search, there is also Algolia DocSearch. I've had a little bit of trouble with it in the past, but think it's overall fine. No idea if it's going to give users cookies.

[ivirshup]

Closing, since it was fixed in #1166

[ayushnoori]

Great, thanks so much, @ivirshup!