docs(gcp): document gcp workload identity federation auth path

[kamil-filipek]

Summary

Changes

Testing

• Tested locally with npm start
• All pages render correctly
• Images display properly
• Internal links work
• Sidebar navigation works

Quality Checks

• npm run check passes (typecheck + lint + commitlint)
• No MDX compilation errors
• No raw angle brackets (<text> must be `<text>`)
• Sidebar references document IDs (not filenames)
• Images stored locally next to content (not in static/img/)
• Commit messages follow Conventional Commits
• No secrets or credentials in documentation

Additional Notes

[github-actions]

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[sharbuz]

LGTM

[elluvium]

Does that mean we're keeping legacy option? If so then I do not think that this is a breaking change in release notes. Let's either remove legacy option or remove breaking change announcement in release notes

[codemie-ai]

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