Docs: add dark theme support

[mlmoravek]

Description:

Add a dark/light mode switcher to the top of the documentation and force the use of the dark/light theme presets.

Why does Oruga need this feature?

To enhance professionalism and showcase themes in dark mode.

[mlmoravek]

Hey @hiSandog, the current problem is that not every theme supports a dark theme yet.
Bootstrap and Bulma has a dark mode. But the default oruga theme does not have a dark mode implemented yet.
This is required first, to enable dark mode for the docs page.