Flowable Open Source Documentation should support dark mode

**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