docs: installation: source: gate new build options to v5.1.3

[eschabell]

Neither FLB_PROTOBUF_ENCODER nor FLB_OUT_ARVANCLOUD_CLOUDLOGS exists in
v5.1.2 or any earlier tag. Both ship in v5.1.3. CMake accepts unknown
-D definitions silently, so passing either on a current release fails
without any diagnostic.

• Annotate the FLB_PROTOBUF_ENCODER build options row and scope the
  note that follows the table.
• Scope FLB_OUT_ARVANCLOUD_CLOUDLOGS in the sentence introducing the
  output plugins table.

Both tables are hand aligned at fixed row widths. The protobuf
annotation is absorbed from that row's existing cell padding, so the
build options table stays at 186 characters per row. The ArvanCloud
row has only 16 characters of padding, so it is scoped in prose rather
than re-padding roughly 90 rows.

Refs #2727

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Advanced

Run ID: 466336d1-5683-43f4-98a2-6573c7306421

📥 Commits

Reviewing files that changed from the base of the PR and between 82cb964 and 5a523fc.

📒 Files selected for processing (1)

• installation/downloads/source/build-and-install.md

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📝 Walkthrough

Walkthrough

The build and install guide now states that FLB_PROTOBUF_ENCODER and FLB_OUT_ARVANCLOUD_CLOUDLOGS are available in Fluent Bit 5.1.3 or later. The documented Protobuf dependencies and compiler requirements remain unchanged.

Changes

Build option availability

Layer / File(s)	Summary
Update option version notes installation/downloads/source/build-and-install.md	The guide states that FLB_PROTOBUF_ENCODER and FLB_OUT_ARVANCLOUD_CLOUDLOGS are available in Fluent Bit 5.1.3 or later. The Protobuf requirements remain documented.

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~4 minutes

Change: Other

Merge Risk: ⚪ Minimal · up to 5a523

The notes steer v5.1.2 users away from options absent in that release. Later-version support was not independently confirmed, but no contradictory evidence or concrete user-facing failure was established; no demonstrated merge-blocking risk remains.

Architecture Summary

Architecture risk: 🔵 Low · up to 5a523

The change affects 1 system.

Changed systems: installation

Architecture concerns
No architecture-level concerns identified.

Review details

Systems and components

• observed — installation (service) was modified; 1 changed file maps to changed impact.

Before / after behavior

• observed — Modified behavior in installation/downloads/source/build-and-install.md: The FLB_PROTOBUF_ENCODER table description now states that the option is supported in Fluent Bit 5.1.3 or later.
• observed — Modified behavior in installation/downloads/source/build-and-install.md: The availability note now says Fluent Bit version 5.1.3 and greater; the documented Protobuf 3.12+, libprotoc requirement and failure message, and C++11/compiler requirement are unchanged.
• observed — Modified behavior in installation/downloads/source/build-and-install.md: The output-plugin introduction now states that FLB_OUT_ARVANCLOUD_CLOUDLOGS is available in Fluent Bit version 5.1.3 and greater.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
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.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the documentation change that limits new build options to Fluent Bit v5.1.3 and later.

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[eschabell]

@patrick-stephens ready for review!