Embed JS: fix incompatibilities with sphinx 6.x (jquery removal)

[stsewd]

• $.urlencode was an alias for encodeURIComponent, use that instead.
  https://github.com/sphinx-doc/sphinx/blob/b26bf0320cf3ec8a3400a70b2dc481d511ff2563/doc/_themes/sphinx13/static/_sphinx_javascript_frameworks_compat.js#L33-L33
• We were using underscore.js just for the template functionality, so I just replaced that with the DOM API.
• JQuery isn't included by default on sphinx, so we are injecting it if isn't present. We should migrate all of our code to be jquery free, but this is a first step.

Our theme still doesn't work becuase of readthedocs/sphinx_rtd_theme#1299.
But this PR doesn't depend on the theme, it can be merged as is.

ref sphinx-doc/sphinx#10574

[humitos]

I think this code is fine. However, if I understand correctly, it seems there are some edge cases we won't be handling that I suppose will break in production due to the order of the scripts loaded.

[humitos]

What's up if this line is executed first and then jQuery is loaded by a different extension/theme/etc?

I'm thinking about this scenario;

1. this code is run and we inject jQuery
2. another .js file is loaded that includes jQuery
3. once domReady is called, we will have 2 different jQuery versions/files loaded

Shouldn't we inject jQuery synchronously inside domReady so we wait for all the scripts to be loaded first and avoid this scenario?

[stsewd]

This is going to be a problem anywhere we try to inject jquery, since the code to be executed after jquery is done loading is inside an asynchronous callback (onload). I think the only difference would be that the page won't be blocked till jquery is done downloading.

[mgeier]

Is it really necessary to inject jQuery at this point?

Sphinx still has jQuery, it is only planned be removed in Sphinx 6.0: sphinx-doc/sphinx#10070

Note that https://www.sphinx-doc.org/ uses the master branch, with jQuery already removed, but they have added the JS frameworks to their docs already: https://github.com/sphinx-doc/sphinx/blob/master/doc/conf.py#L181-L186, so it shouldn't be necessary to add it from the RTD side.

[stsewd]

Is it really necessary to inject jQuery at this point?

There are some themes on mkdocs that don't include jQuery (material).

[stsewd]

I have updated this PR to not inject jquery yet, so isn't blocked on that, this still removes some dependency on jquery.

[agjohnson]

This is a good refactor, it makes a lot of sense to chip away at jQuery removal like this