backport pre-release docs handling

[d-morrison]

see d-morrison/gdl#2

[d-morrison]

@claude

[claude]

Claude finished @d-morrison's task in 7m 55s —— View job • claude/issue-135-20260519-0322 • Create PR ➔

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Backporting pre-release docs handling

• Read CLAUDE.md for repo-specific instructions (no CLAUDE.md found)
• Explore repository structure
• Analyze current code to infer changes needed (direct PR access unavailable due to permission restrictions on external repos)
• Implement pre-release docs handling in generate_version_dropdown.py
• Commit and push changes

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Changes made to .github/scripts/generate_version_dropdown.py:

1. Added _is_prerelease_version(version) — detects R development versions (4-component where the 4th component ≥ 9000, e.g. 1.2.3.9000)

2. Conditional dev entry — the (dev) dropdown entry is now only shown when origin/main:DESCRIPTION actually contains a pre-release version. Previously, a (dev) link was always emitted even when main held a stable release version (e.g. right after tagging before a version bump), which would create a misleading dropdown entry.

3. Pre-release tag detection — R-style 4-component tags (v1.0.0.9000) and suffixed tags (v1.0.0-beta.1, v1.0.0-rc1) are now identified and the most recent one is shown as a (pre-release) entry in the versions dropdown, between the stable entry and the dev entry.

Note: I was unable to access the referenced PR at d-morrison/gdl/pull/2 directly (external gh/curl/network commands require approval in this environment). The implementation above reflects the most natural interpretation of "pre-release docs handling" for an R package altdoc site. Please review and let me know if adjustments are needed to match the gdl PR exactly.