Add more/better examples for custom rules to the documentation #1438

The user guide only has one example for an ArchCondition in its section on Creating Custom Rules.

In our project we have made the experience that this example doesn't seem to be optimal:

• The easiest way to create a condition seems to be implementing a DescribedPredicate and then using ArchConditions.be or ArchConditions.have to convert the predicate to a condition. However, this mechanism seems to be entirely absent from the user guide.
• Using SimpleConditionEvent.violated means that the condition cannot be inverted using ArchConditions.not. Wouldn't it be better to use new SimpleConditionEvent and pass call.getOrigin().isAnnotatedWith(Secured.class) to the event's conditionSatisfied parameter instead?