[feature] Docs: Dark Mode

[KCarretto]

Enable a dark mode toggle in the nav bar for our docs. It should persist the setting, such that subsequent page visits or reloads preserve the user’s choice.

[hulto]

Adding a dark mode button to the existing theme will be hard seems like it will require a JS element to handle the toggle and then building a new style sheet which could be tedious.
If we switch to a just the docs theme (https://just-the-docs.github.io/just-the-docs/docs/configuration/#color-scheme) we can take advantage of the jtd.getTheme() and jtd.setTheme()

[hulto]

Here's a PoC of JTD with a light / dark mode button.

[hulto]

Closing for now.
Will reopen if an acceptable approach is found with doxy.