Add example for adding xrefs

[cthoyt]

I would like to see two additional examples/functionalities in https://incatools.github.io/kgcl/examples/:

1. Adding an xref. Is it also possible to add extra metadata like the contributor ORCID with an xref?
2. Updating an xref. E.g., I know there's already an xref from A to B with a given relationship like oboInOwl:hasDbXref and want to upgrade it to a more specific relation like skos:exactMatch

It's also not clear from the docs what happens if I try upgrading an xref that doesn't exist. E.g., if I try and upgrade A oboInOwl:hasDbXref B to A skos:exactMatch B but A oboInOwl:hasDbXref B doesn't exist, will it still add A skos:exactMatch B?

Motivation: I want to automate adding Biomappings to various ontologies