Fill gaps in gantt diagram documentation

[GDFaber]

The documentation for gantt diagrams has some sections which are marked "to be done". The missing parts should be added to the docs.

I also added styling/hiding the "today line" by adding todayMarker to the gantt definition in #1336. This feature is also undocumented at the moment and should be added here as well.

[CollierCZ]

I can give this a try.

[GDFaber]

Hi @CollierCZ, that's great. Our contribution guide can be found here. Please feel free to ask if you need additional information.