Optional explicit ordering of examples

[david-christiansen]

Talking with @VictorCMiraldo at lunch today about #436, we came to the conclusion that error docs should support optional ordering of examples (when the one writing the documentation has a particular idea about which examples are most likely to be relevant). The design we hit on would be extending the YAML block in an example's index.md to allow an optional field order that should contain an int. Examples can then be ordered by this field. Those without an order would come at the end (so absence is negative infinity) and anything with the same explicit order field would be displayed in some arbitrary order within the group.

[VictorCMiraldo]

Indeed, I'll give a quick stab at this before leaving Zurich!