Make yaml/xml configuration for  "AttributeOverride" and "AssociationOverride" more discoverable

I was trying to find out how to configure `AttributeOverride` in yaml for one of my entities and all paths led me to the [Override Field Association Mappings In Subclasses](http://docs.doctrine-project.org/projects/doctrine-orm/en/latest/tutorials/override-field-association-mappings-in-subclasses.html) tutorial.

It was a lot harder to find anything for xml/yaml. This pull request adds a link to the [Overrides section](https://doctrine-orm.readthedocs.org/en/latest/reference/inheritance-mapping.html#overrides) of the inheritance mapping reference which provides the needed examples.

Alternatively yaml/xml examples could be provided in the tutorial but I am not familiar enough with sphinx/the docs to make those changes