docs: installation: downloads: linux: raspbian: use signed-by for Bullseye apt source

[eschabell]

• Replace apt-key add with gpg --dearmor | sudo tee to store key in /usr/share/keyrings
• Add signed-by=/usr/share/keyrings/fluentbit-keyring.gpg to Raspbian 11 (Bullseye) apt source entry
• Wrap Bookworm, Bullseye, and Buster codenames in backticks in section headings

Replaces #1147

Summary by CodeRabbit

• Documentation
  • Updated Raspberry Pi installation guide with improved GPG key setup procedures
  • Added separate installation instructions for Raspbian 12, 11, and 10 versions
  • Enhanced clarity for APT repository configuration

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: c7aa9559-410f-453d-800e-e69c0f1cb7c7

📥 Commits

Reviewing files that changed from the base of the PR and between 201b3e9 and 4507ad0.

📒 Files selected for processing (1)

• installation/downloads/linux/raspbian-raspberry-pi.md

📝 Walkthrough

Walkthrough

This PR updates the Raspberry Pi Fluent Bit installation guide to use modern GPG key management practices. The GPG key is now installed into a dedicated keyring file using gpg --dearmor, and the APT sources configuration is reorganized with separate subsections for each Raspbian version, including a signed-by directive for Bullseye.

Changes

Raspberry Pi Installation Guide Updates

Layer / File(s)	Summary
GPG key installation and APT sources configuration installation/downloads/linux/raspbian-raspberry-pi.md	GPG key installation is updated from apt-key to gpg --dearmor with output to a dedicated /usr/share/keyrings/fluentbit-keyring.gpg keyring file. APT sources list is reorganized with separate subsections for Raspbian 12 (Bookworm), 11 (Bullseye), and 10 (Buster), and a signed-by= directive is added for Bullseye repositories.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Suggested reviewers

• cosmo0920
• patrick-stephens

Poem

🐰 A keyring update, secure and fine,
GPG dearmor'd, one file to align,
Raspbian versions, now each has its place,
Signed repositories, a modern embrace!

🚥 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 accurately reflects the main change: updating the Raspbian installation guide to use signed-by for Bullseye apt source, which directly aligns with the primary objective of adding signed-by=/usr/share/keyrings/fluentbit-keyring.gpg to the Raspbian 11 (Bullseye) apt source entry.
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.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@installation/downloads/linux/raspbian-raspberry-pi.md`:
- Line 26: The apt source lines for Bookworm and Buster are missing the
signed-by directive pointing to the installed GPG keyring; update the echo
commands that create /etc/apt/sources.list.d/fluent-bit.list for Bookworm (the
echo at the Bookworm source) and Buster (the echo at the Buster source) to
include [signed-by=/usr/share/keyrings/fluentbit-keyring.gpg] immediately after
"deb" so they match the Bullseye entry and allow APT to locate
/usr/share/keyrings/fluentbit-keyring.gpg for signature verification.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 8e316b3a-4623-4ce1-903a-8b617499e011

📥 Commits

Reviewing files that changed from the base of the PR and between 5231934 and 201b3e9.

📒 Files selected for processing (1)

• installation/downloads/linux/raspbian-raspberry-pi.md

[eschabell]

@cosmo0920 and @patrick-stephens would you look at this replacement docs PR for the old one?