Documentation for user contribution mapping components

What does this MR do and why?

This MR adds development documents for user contribution mapping allow engineers to quickly familiarize themselves with the user mapping feature and how it works on the backend. Including:

• Glossary of commonly used terms, what backend models they correspond to, and what they mean
• Generic overview of how user mapping is implemented in an importer, and how to update the PlaceholderReferences::AliasResolver
• Overview of how placeholder reassignment works
• How to update Import::PlaceholderReferences::AliasResolver when DB schema for user contributions changes

This MR also updates the logged placeholder references alias warning to include a link to the docs on aliasing.

References

• Resolves #449034 (closed)

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #449034 (closed)