[documentation] Document support for multiple conditions, including support for "aggregate"

[hydrosquall]

Motivation

• The "array" form of conditions was added in Support multiple conditions in an encoding (if/else/else...) #3040 . However, none of the examples on this page use this format for showing conditions in action.
• Related bug: conditions are converted from single to array form, the JSONSchema does not like it, even though the object form is ok

Suggested fixes

1. Add an array example to condition page
2. Look into why the aggregate field is marked as invalid per the vega-lite schema

[dangotbanned]

@hydrosquall I recently did some work on the altair support for this in:

• feat: Improve the syntax for conditions with multiple predicates altair#3427
• fix(typing): Improve Then annotations, autocompletion, docs altair#3567

I think the issue you have here is that you can have multiple Conditional Value Definition(s) but only one Conditional Field Definition

I would agree that the Condition docs should demonstrate this more.

Also there is Bar Chart with Highlighting on Hover and Selection on Click in the examples to draw from