Improve documentation of Model and ModelNode usage

[josephbirkner]

Currently, it is hard to figure out how to use and extend Models and ModelNodes. This concept, as well as all existing Node types should be extensively documented (mainly in docstrings for model/nodes.h).

[josephbirkner]

For example, the question came up for how to iterate over the fields of an object. This would be the answer:

for (auto const& [fieldNameId, fieldValueNode] : object->fields()) {
   auto fieldNameStr = *fieldNames().resolve(fieldNameId);
   auto fieldValue = fieldValueNode->value();
   ...
}