docs: installation: consolidate what's new pages into a rolling v5 page

[eschabell]

• Rename whats-new-in-fluent-bit-v5.0.md to whats-new-in-fluent-bit-v5.md and restructure as a rolling page covering the v5 line, newest release first
• Add a v5.1 section covering multi-worker ingestion, input rate gate, FIPS mode, TLS cert reload, tls.proxy.* HTTPS proxy, DTLS syslog output, ETW input, NVIDIA/NVML GPU metrics, node exporter metrics additions, GCS output, Kafka Schema Registry, file output rotation, elasticsearch alias, Windows Server 2025/Nano Server images, and the 64-bit timestamp fix
• Update SUMMARY.md to a single "What's new in Fluent Bit v5" entry
• Add a .gitbook.yaml redirect from the old v5.0 page URL
• Add a Fluent Bit v5.1 section to upgrade-notes.md covering the Debian/Ubuntu restart-on-upgrade behavior, syslog mode: tls/dtls, and the 64-bit timestamp fix

Summary by CodeRabbit

• Documentation

  • Added a consolidated Fluent Bit v5 overview covering v5.0 and v5.1 features, performance improvements, security, integrations, configuration, monitoring, and packaging.
  • Added v5.1 upgrade guidance, including restart requirements, Syslog TLS modes, extended timestamp support, and updated examples.
  • Updated navigation, upgrade links, and redirects to the renamed v5 documentation.

• Style

  • Improved documentation linting and editorial consistency for headings, acronyms, and spelling.

[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: 4cb31861-2d34-4794-9668-1306dd263f5d

📥 Commits

Reviewing files that changed from the base of the PR and between df5b6df and 64f4683.

📒 Files selected for processing (10)

• .gitbook.yaml
• .markdownlint.json
• README.md
• SUMMARY.md
• installation/upgrade-notes.md
• installation/whats-new-in-fluent-bit-v5.0.md
• installation/whats-new-in-fluent-bit-v5.md
• vale-styles/FluentBit/Acronyms.yml
• vale-styles/FluentBit/Headings.yml
• vale-styles/FluentBit/Spelling-exceptions.txt

💤 Files with no reviewable changes (1)

• installation/whats-new-in-fluent-bit-v5.0.md

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📝 Walkthrough

Walkthrough

The PR replaces the Fluent Bit v5.0 release page with a combined v5 overview, adds v5.1 upgrade notes, updates documentation links and redirects, and adjusts Markdownlint and Vale rules for new terminology.

Changes

Fluent Bit v5 documentation

Layer / File(s)	Summary
Combined v5 release overview installation/whats-new-in-fluent-bit-v5.md, installation/upgrade-notes.md	Adds v5.1 and v5.0 release details, upgrade notes, configuration changes, security features, integrations, and timestamp behavior.
Documentation links and redirect .gitbook.yaml, README.md, SUMMARY.md	Redirects the former v5.0 path and updates v5 documentation links to the renamed page.
Markdown and Vale validation .markdownlint.json, vale-styles/FluentBit/*	Configures sibling-only duplicate-heading checks and adds exceptions for new acronyms, spellings, and heading capitalization.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Possibly related PRs

• fluent/fluent-bit-docs#2528: Updates the same upgrade notes with Fluent Bit v5 guidance.
• fluent/fluent-bit-docs#2394: Adds version-specific upgrade and breaking-change guidance.
• fluent/fluent-bit-docs#2167: Updates the same Vale exception files.

Suggested reviewers: patrick-stephens, cosmo0920

🚥 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 summarizes the main change: consolidating the Fluent Bit v5 release documentation into a rolling page.
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!