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