docs: set up versioned documentation with mike

[MaxMichel2]

Summary

• Adds mike-based versioned documentation via Zensical's version provider integration
• Fixes the installation page showing 0.2.0 instead of the published 0.1.3 — mike deploys from the tag commit where VERSION_NAME has no -SNAPSHOT suffix
• Fixes stale What's New section — release.py now auto-updates docs/index.md from the changelog on every release

Changes

• zensical.toml: enables the version switcher dropdown (provider = "mike", alias = true)
• scripts/build_docs.sh: reads version from DEVVIEW_VERSION env var first (falling back to gradle.properties), and adds --prep-only flag so CI can pre-process docs without running zensical (mike handles the build)
• .github/workflows/publish-docs.yml: restructured for mike — resolves version from VERSION_NAME (SNAPSHOT → dev, release tag → version + latest alias), runs mike deploy, cleans up old versions; adds workflow_dispatch inputs for manual deploys
• scripts/release.py: new step 3b auto-updates docs/index.md What's New from the release changelog section
• docs/index.md: What's New updated from v0.1.2 to v0.1.3 (one-time fix)

After merging

1. Switch GitHub Pages source to "Deploy from a branch" → gh-pages / / (root) in repo settings
2. Trigger workflow_dispatch with version: 0.1.3, aliases: latest to seed the first versioned deployment for the published release
3. Subsequent pushes to main will auto-deploy as dev; future release tag pushes will auto-deploy as the release version with latest alias

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