docs: expand repository scan examples (current dir, branch, commit)

[jsibitoye]

Description

This PR expands the examples in the trivy repository command docs.

• Added example for scanning the current working directory (trivy repo .)
• Added example for scanning a specific branch (--branch develop)
• Added example for scanning a repository at a specific commit (--commit <hash>)

These additions give new users quick, practical commands for common scenarios without changing any functionality.

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Checklist

• [*] I've read the guidelines for contributing to this repository.
• [ *] I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

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[jsibitoye]

This PR expands trivy repo usage examples (dir, branch, commit) and adds CI/CD integration references, making docs more practical and user-friendly.