Update project import documentation

[loglapa]

What changed

• Replace the removed gitlab:import:repos examples with gitlab:import_export:import.
• Document the four required import parameters and the expected project export archive.
• Provide commands for docker run, docker exec, and stopped or running Compose services.
• Link to the current GitLab project import/export Rake task documentation.

Why

GitLab removed bare repository importing and the gitlab:import:repos Rake task in 16.0. The existing README instructions therefore fail on current docker-gitlab releases.

The supported replacement imports a project export archive created by a compatible GitLab instance.

Impact

Administrators following the README get a working command and the required parameters instead of invoking a task that no longer exists.

Validation

• git diff --check
• Confirmed gitlab:import:repos no longer appears in README.md
• Confirmed all replacement examples use gitlab:import_export:import
• Confirmed the linked GitLab documentation is reachable

Fixes #3040