Document steps for importing libraries from popular management software

Is your suggestion for improvement related to a problem? Please describe.
Migrating to/from JabRef from/to Zotero, Mendeley, Qiqqa can be very cumbersome with enough groups/folders/tags in whichever of the platforms.

Describe the solution you'd like
Implement any needed basic functionality for migrating common feature sets in an easy-to-write format.

https://discourse.jabref.org/t/migrate-mendeley-folders-into-jabref-subgroups/2007

Then document the steps needed to construct those migration files from a number of popular alternative software (e.g. Zotero, Mendeley).

For Mendeley, will probably need a custom module for the migration :/ Not sure about Zotero.

Additional context
The current "Import" dialog doesn't require any specific file types, but does not seem to support Mendeley zip backups. I'm guessing it supports only .bib files. Should make that clearer.

Start with the linked Discourse discussion and inspect JabRef's current Import dialog and supported file types. The requested work spans migration support and documentation for Zotero, Mendeley, and Qiqqa, including whether Mendeley ZIP backups are supported; define the migration scope before documenting the completed steps.