Implement dark-mode

[zoglo]

Dark-mode

• This PR introduces a dark-mode toggle below the search bar and listens to prefers-color-scheme: dark as well
• Light mode will stay the same for everyone, I did not change any colors
• Dark-mode colors are mostly the same as Contao 5
• Dark-mode option is saved in local storage similar to Contao 5
• Using 'data-theme=dark' also uses most dark-themes (e.g. Algolia)
• I couldn't change the pictures (They are still in light mode)

Preview

Can be temporarily accessed through these two links: removed

[zoglo]

I found one more thing that has been missing in this PR and did implement it in my last commit: https://github.com/contao/docs/pull/1301/commits/27d6d00d2951038c78f62c75990c963d34c73ba2.

This also adds dark mode for mermaid.js but only works when reloading the page due to how mermaid.js works.

Examples

Light mode

Dark mode

The PR should be ready for review @fritzmg @netzarbeiter