doc: add some text regarding libyang versions

[choppsv1]

No description provided.

[greptile-apps]

Greptile Summary

This PR updates the libyang build documentation to provide clearer, more comprehensive guidance on which libyang versions to use with FRR:

• Modernizes the introductory text, removing outdated "relatively new" language
• Adds a prominent note block documenting version guidance: v2.1.128 minimum required, v3 recommended (supported since FRR 10.2), v4 supported but not preferred due to LYD_LYB binary encoding restrictions, and v5 as the future preferred version
• Moves the v2.1.128 minimum requirement from a warning under "Option 1: Binary Install" to the top-level note for better visibility
• Updates the source install example from git checkout v2.1.128 to git checkout v3.12.2, consistent with the v3 recommendation
• Adds a note clarifying the checkout tag is a suggestion, not a strict requirement

The version guidance is consistent with configure.ac, which enforces libyang >= 2.1.128 and emits a warning for v4 about LYD_LYB format restrictions.

Confidence Score: 5/5

• This PR is safe to merge — it only changes documentation with no code impact.
• This is a documentation-only change to a single RST file. The version guidance is accurate and consistent with the project's configure.ac. No code logic is affected, and the updated build instructions correctly reflect the project's current libyang version recommendations.
• No files require special attention.

Important Files Changed

Filename	Overview
doc/developer/building-libyang.rst	Documentation update: modernizes intro text, adds comprehensive libyang version guidance (v2.1.128 min, v3 recommended, v4 caveats, v5 future), updates source install example to v3.12.2, and adds flexibility note about the tag choice. All version claims are consistent with configure.ac.

Flowchart

flowchart TD
    A["Building FRR"] --> B{"Check libyang version"}
    B -->|"< 2.1.128"| C["Build fails: version too old"]
    B -->|">= 2.1.128 (v2.x)"| D["Supported (minimum)"]
    B -->|"v3.x"| E["Recommended"]
    B -->|"v4.x"| F["Supported, but not preferred\n(LYD_LYB restrictions,\nfalls back to JSON/XML)"]
    B -->|"v5.x"| G["Future preferred\n(LYD_LYB restrictions removed)"]
    D --> H["Build FRR"]
    E --> H
    F --> H
    G --> H

Loading

_{Last reviewed commit: 46bb688}

[mjstapp]

thanks, this is helpful. had a couple of questions

[Jafaral]

@Mergifyio backport dev/10.6

[mergify]

backport dev/10.6

✅ Backports have been created

Details

• #20923 doc: add some text regarding libyang versions (backport #20862) has been created for branch dev/10.6

[choppsv1]

https://github.com/Mergifyio backport stable/10.5 stable/10.4 stable/10.3

[mergify]

backport stable/10.5 stable/10.4 stable/10.3

✅ Backports have been created

Details

• #20924 doc: add some text regarding libyang versions (backport #20862) has been created for branch stable/10.5
• #20925 doc: add some text regarding libyang versions (backport #20862) has been created for branch stable/10.4
• #20926 doc: add some text regarding libyang versions (backport #20862) has been created for branch stable/10.3