Add dark mode support to website

[LouisDISPA]

Hello, I quickly did a dark mode to the website just for fun and see how complex it would be. This is a very simple implementation only in css. Feel free to close this PR if you don't want this feature or don't like it.

I'm happy to work more on it if you are interested in this feature like:

• cleanup CSS variables
• add a theme switcher button (would require some javascript)
• test different colors

List of changes:

• Use CSS prefers-color-scheme media queries
• Create dark version of the logo for dark mode
• Code block syntax highlighting
• Syntax highlighting dark mode support
• Update Zola to v0.20
• Add hover state for links

Screenshots:

[sylvestre]

this is excellent, thanks!

[tertsdiepraam]

Looks excellent! I like the implementation with CSS variables too. Any reason to keep this as a draft?

[LouisDISPA]

Sorry for the late response.
I put this as a draft to see if you were interested in this.
Thanks you very much for the feedback!

I fixed up a bit the variable and also propose to capitalize the menu and titles for consistency.
I can revert the last commit if you don't like it.

I tested all pages in light and dark mode, I think it's safe now.
You can merge this PR like this it !