Update developer documentation of importers to describe User Contribution Mapping

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We have some existing development guidelines documentation of our importers that should be updated to reflect how User Contribution Mapping works in each importer.

For each importer, add a section that concisely details for another developer what they need to know in order to hook up User Contribution Mapping of new objects/relations. For example, how they would:

• Map to source users
• Push placeholder references
• Any other relevant UCM information for that importer

For these importers with existing documentation:

• https://docs.gitlab.com/ee/development/bitbucket_cloud_importer.html
• https://docs.gitlab.com/ee/development/bitbucket_server_importer.html
• https://docs.gitlab.com/ee/development/github_importer.html
• https://docs.gitlab.com/ee/development/bulk_import.html
• https://docs.gitlab.com/ee/development/import_export.html (At time of writing this importer doesn't support UCM)