docs: add Git and GitHub onboarding documentation

- Add git-fundamentos.md with basic and intermediate Git concepts
- Add github-colaboracao.md with GitHub workflow and collaboration
- Include Mermaid diagrams for visual learning
- Update roteiro-onboarding.md to integrate new docs into learning paths
- Update mkdocs.yml navigation

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: nitaibezerra