This updates the furo docs theme to the latest version.

The main intention however is bumping Sphinx to 6.x, which drops the unnecessary jQuery+underscore.js bloat:
https://www.sphinx-doc.org/en/master/changes.html#release-6-0-0-released-dec-29-2022

Sphinx's minimum version requirement however is 5.x (defined by furo and the other build dependencies). I decided against bumping the min version to 6.x, because Sphinx is also used for building the man page. Some Linux distros are still on Sphinx 5 and I don't want to unnecessarily introduce incompatibilities for no reason.

Sphinx 6 also drops support for py37, but we've already dropped support for building docs on py37 a while ago in 46496db

In order for being able to bump Sphinx to 5.x/6.x, myst-parser and sphinx-design needed an upgrade. Those have finally been published a couple of weeks and days ago respectively:

Bumping furo also required some fixing due to changes of the theme's logo variable.

I couldn't spot any issues after the upgrade, but please check and compare the netlify preview build first.
https://deploy-preview-5301--streamlink.netlify.app/