feat: import existing projects (instructions)

[geritwagner]

Feature Request

The docs should provide instructions on how to import existing projects.

• Existing projects can have separate bibliography files for included/excluded papers. To import such projects, we should implement a new option colrev screen --auto_include filepath, which automatically includes (and prescreen-includes) all records from the filepath SearchSource.
• Alternatively, a screening table could be registered for automated inclusion/exclusion based on the origin IDs in the SearchSource.
• PDFs may need to be imported/linked
• for duplicates: could set to md_processed automatically/manually
• It may be helpful to prevent changes in record IDs (especially when records have progressed to the data operation).