feat: enable dark theme

[raulb]

Enabling dark theme in docusaurus is trivial (set this to false), but we need to update the images and styles so they still look good:

• Refer to data-theme="dark" or data-theme="light"

[hariso]

Looks nice to me +1. It looks like the diagrams are a bit less readable in the dark mode, probably because they're transparent.

[raulb]

Here's a branch to whoever picks it up https://github.com/ConduitIO/conduit-site/tree/enable-dark-theme.