Improve visual structure of "union" parameters

[soxofaan]

this came up at Open-EO/openeo-processes#375

the docs of the condition parameter of the count process are hard to process visually:

it takes a trained eye to see that condition allows three options: a process graph, value true (note tat false is illegal) and value null

[soxofaan]

some brainstorming on how to improve this:

• a lot of horizontal space is wasted, can we structure things more horizontally to make this listing less long visually
• make process graph parameter listing collapsable (and collapsed by default), to avoid confusing count's parameters with the the condition process graph parameters?

[m-mohr]

As a quick fix I've just tried to separate them a bit better through some styling changes and also by making the allowed value for booleans more clear: