Add _base to documentation

[RichDijk]

I have spent many hours solving a @base prefix issue before I came tot the notion of _base The message I got was:

Unknown CURIE prefix: @base 

generated from namespaces.py

rdflib_dumper.py states:

    # user can pass in base in prefixmap using '_base'. This gets set
    # in namespaces as a plain dict assignment - explicitly call the setter
    # to set the underlying "@base"

This is an issue every initial user of linkml generating rdf will experience. This first experience is important for adoption.

[giovannidegani]

Thanks for this issue, I have spent a few hours trying to figure out rdflib_dumper was giving the Unknown CURIE prefix: @base error