docs: pipeline: outputs: gcs: add Google Cloud Storage output plugin page

[eschabell]

Document the out_gcs plugin introduced in Fluent Bit 5.1, including all configuration parameters, service account authentication, object key format, and local buffering behavior. Add the page to SUMMARY.md and allow "Google Cloud Storage" in the Vale heading capitalization exceptions.

Note: covers missing doc PR for a code change.

Summary by CodeRabbit

• Documentation
  • Added comprehensive documentation for the Google Cloud Storage output plugin, including prerequisites, credentials, supported events, configuration options, buffering, object naming, and usage examples.
  • Added Google Cloud Storage to the outputs documentation table of contents.
  • Updated heading style guidance to correctly preserve the plugin name’s capitalization.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 07897db7-a8af-4b13-81a7-535631180be5

📥 Commits

Reviewing files that changed from the base of the PR and between 0eb06a6 and 6f2e5e8.

📒 Files selected for processing (3)

• SUMMARY.md
• pipeline/outputs/gcs.md
• vale-styles/FluentBit/Headings.yml

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📝 Walkthrough

Walkthrough

This change adds Google Cloud Storage output plugin documentation, links it from the outputs table of contents, and adds its product name to the Vale heading capitalization exceptions.

Changes

Google Cloud Storage documentation

Layer / File(s)	Summary
Document and validate Google Cloud Storage output pipeline/outputs/gcs.md, SUMMARY.md, vale-styles/FluentBit/Headings.yml	Adds setup, authentication, configuration, formatting, buffering, and example details. Adds the table of contents link and heading capitalization exception.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Suggested reviewers: patrick-stephens

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely identifies the documentation page added for the Google Cloud Storage output plugin.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[eschabell]

@patrick-stephens ready for review!