Add simple SOPs for updating imports to CL doc

[dosumis]

In discussion with @bvarner-ebi it has become clear that the general oBook documentation on imports does not work well as a guide for updating imports no CL. Users need to know beforehand what strategy is used for imports and the guide (apparently) lacks the fine grained detail needed to successfully run the pipeline for CL. In addition, some imports (e.g. PR) need to be treated differently because of the size of the upstream ontology.

This thread illustrates the problem well - and has the beginnings of a more workable SOP for CL imports.

TODO I propose that @bvarner-ebi write a short draft SOP for inclusion in CL doc. @anitacaron and @shawntanzk can review before incorporating into the standard auto-generated doc.

Big picture: Centralised documentation in the oBook is great, but generalisation comes at a cost to usability. I think local SOPs for ontologies are therefore unavoidable. We also need them ASAP for new curators.

[github-actions]

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[ghost]

Instructions can be found here: https://github.com/obophenotype/cell-ontology/blob/master/docs/Adding_classes_from_another_ontology.md