Please add details on "mirrors" as they relate to refreshing imports

[ghost]

As briefly discussed in https://github.com/obophenotype/cell-ontology/pull/1644, I am requesting a section on "mirrors" to be added or linked to the Update Imports Workflow page.

A novice editor may benefit in the following:

• Knowing what a "mirror" is
• Understanding how a mirror relates to ontologies and imports
• Guidance on how to make decisions at points in the instructions that mention mirrors. E.g., "If you wish to skip refreshing the mirror, ..." - what must an editor consider in deciding whether or not to skip refreshing a mirror? "Note: if your mirrors are updated, ..." - how does an editor determine whether or not the mirrors are updated?

Thank you for your considerations.