adapt docs to the cedardb CI

[lukasvogel]

Just make the docs seem a touch more professional and in tune with the cedardb CI from the main website.

Before:

After:

[Lukasdoe]

I really like this new design! Just a few points that I cam currently missing:

• Link hover effect (maybe background to orange)
• Codeblock on first page is very nice, but needs a bit more structure (currently more like a blob of SQL) and needs rounded corners (to look like the other code blocks)
• Emojis in callouts are not on line of callout text (invalid vertical offset)
• Side menu button to expand the menu point into its submenu is a bit small and hard to click (just an opinion)
• The index pages (e.g. /docs/cookbook/) still need some more love, since they are currently very hard to navigate (lots of unstructured text)
• Inline tables (when width < textwidth) are left aligned. That somehow breaks the flow a bit. Maybe we should center them or stretch them to page width.
• Nice to have: If you're already at it, you could add syntax highlighting for psql codeblocks.

All of them are not mandatory and could very well be fixed in the future.