Switch theme (again)

[koppor]

https://docusaurus.io/ seems to support our wishes best.

This would fix #68 and remove the dirty hacks.

Demo: https://webdriver.io/docs/contribute - and I think, this is the layout we need: on top the different sections - and on the right the section headings.

Moreover, docusaurus is more maintained than chirpy and just-the-docs.