Add example for adding xrefs

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

Start with the examples at https://incatools.github.io/kgcl/examples/ and review the existing xref operations and their documented behavior. Add examples for creating an xref with contributor ORCID metadata and updating an existing xref, and document what happens when the source xref is absent.

Written by the indexing model from the issue text.