Fix defaults in datasource gitlab_projects

Summary

This change improves the documentation and default value handling for three configuration options in the GitLab projects data source: max_queryable_pages, page, and per_page.

Previously, the default values for these settings (10, 1, and 20 respectively) were defined in the code but not mentioned in the documentation, making it unclear what would happen if a user didn't specify them. Now, the defaults are clearly stated in the documentation, and the code has been updated with a small helper function (getValueOrDefault) that explicitly applies these defaults when a user leaves the settings unset — rather than relying on commented-out framework-level defaults.