Generate and publish KDoc via Dokka + GitHub Pages

[halotukozak]

Summary

• Adds the Dokka Gradle plugin (v2.2.0) to core and plugin, aggregated at the root so ./gradlew :dokkaGenerate produces a single HTML site under build/dokka covering both modules, with source links back to GitHub.
• Adds .github/workflows/docs.yml, which builds the docs and deploys them to GitHub Pages on every push to master (also runnable manually via workflow_dispatch).
• core's Maven Central javadoc jar is now auto-derived from Dokka instead of being empty (plugin keeps JavadocJar.Empty(), which is standard for Gradle plugins).
• Adds an API docs link to the README.

Post-merge setup (one-time, needs repo admin)

GitHub Pages must be enabled before the workflow can actually publish anything:

1. Go to Settings → Pages on this repo.
2. Under Build and deployment → Source, select GitHub Actions.
3. Re-run the Docs workflow (or push to master) if it already ran and failed on the deploy step before Pages was enabled.

Once enabled, docs will be live at https://avsystem.github.io/justworks/ and stay up to date automatically on every merge to master.

Test plan

• ./gradlew :dokkaGenerate succeeds locally, produces build/dokka/index.html aggregating both modules
• ./gradlew ktlintCheck passes
• docs.yml workflow succeeds on GitHub Actions (needs Pages enabled first, see above)

[codecov-commenter]

Welcome to Codecov 🎉

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