User mapping - Add documentation on how the feature works

Add a section to our development docs about how user mapping works.

The section should be linked from here https://docs.gitlab.com/ee/development/feature_development.html#import-and-export.

There's a flow diagram in https://gitlab.com/gitlab-com/gl-security/product-security/appsec/appsec-reviews/-/issues/236.

We should also describe the code classes involved.

Some good examples of developer documentation include:

• https://docs.gitlab.com/ee/development/github_importer.html
• https://gitlab.com/gitlab-org/manage/import-and-integrate/team/-/blob/main/integrations/slack.md