Include jquery in the theme

[stsewd]

Sphinx is removing jquery on the 6.x version.

I have avoided using jquery before the main call to the theme, and on the main call from the theme load jquery if isn't present.

I have removed the require statement, since the jquery object is global, and could not be defined at the start of the script.

Related: https://github.com/readthedocs/sphinx_rtd_theme/issues/1253