Clarify IdP group mapping vs invites

[KiraLempereur-Spacelift]

Description of the change

Added details for IdP group mapping for user management, as well as a link to the full IdP group mapping page under Authorization. Noted that it's an either/or situation, and you don't need to invite users if you're using IdP group mapping.

Checklist

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Note

Low Risk
Docs-only changes (no product behavior) with minimal risk beyond potential confusion if instructions/links are incorrect.

Overview
Documentation update clarifying that access can be granted either via direct user invites or via IdP group mapping, and that invites aren’t required when IdP group mapping is used.

Expands docs/concepts/user-management/README.md with a new IdP group mapping section including UI steps, role/space binding details, and an important note that manually granted higher permissions override group-mapped permissions; also adds a link from docs/getting-started/invite-teammates/README.md pointing readers to IdP group mapping as an additional onboarding option and fixes a minor link anchor.

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