Add documentation for adding new catalogers

Cataloger objects are the foundation to how syft understands how to parse sources, discover files, and reveal packages. We should add explicit documentation in a `DEVELOPING.md` guide on the high level steps that are needed to add new catalogers for more package sources.

Start by reviewing the cataloger concepts described in the issue and the repository's existing cataloger implementation, then use DEVELOPING.md as the destination for the guide. Document the high-level steps needed to add a cataloger for a new package source; done means a newcomer can follow the guide to understand the required workflow.

Written by the indexing model from the issue text.