Flowable Open Source Documentation should support dark mode

[abccbaandy]

Is your feature request related to a problem? Please describe.
As title

Describe the solution you'd like
Provide a button to switch to dark mode

Describe alternatives you've considered
Workaround is use github version here, since github have dark mode support.
https://github.com/flowable/flowable-engine/tree/main/docs/docusaurus/docs