[chore] Set up GitHub Pages for rimba docs site

[lugassawan]

Type

• Documentation (README, docs/, command help text)
• CI / workflow (.github/workflows/, actions)
• Dependency / tooling bump (gomod, lint, goreleaser)
• Repository config (templates, Dependabot, labels)
• Tests (coverage gaps, e2e, fixtures)
• Release / packaging (.goreleaser.yaml, scripts/)

Files to change

• .github/workflows/pages.yml (new) — build + deploy workflow
• docs/_config.yml or mkdocs.yml (new, depending on option chosen)
• Possibly restructure docs/ if the chosen tool expects a specific layout

Description

rimba already has substantial markdown ready to publish:

• docs/commands.md (~22 KB) — full command reference
• docs/configuration.md (~5.8 KB) — settings reference
• README.md — project overview and quickstart

Publishing to GitHub Pages turns these into a browsable, searchable site without requiring users to read raw markdown on GitHub.

Three implementation options; Option B is recommended:

Option A — Zero-config Jekyll (minimal effort)
Enable Pages from the main branch /docs folder in repo Settings. Accepts GitHub's default Cayman theme. Smallest possible PR but no sidebar navigation or search.

Option B — Themed Jekyll (RECOMMENDED)
Add docs/_config.yml with the just-the-docs theme (provides sidebar navigation + built-in search). Add a minimal Gemfile. Deploy via a pages.yml workflow using actions/configure-pages + actions/deploy-pages. No non-Go tooling required at runtime; GitHub-hosted runners handle the Jekyll build. Well-supported and polished output.

Option C — MkDocs or Hugo
Richer navigation/search but introduces a non-Go build dependency and ongoing maintenance burden. Not recommended unless the team already uses one of these.

The implementer should choose Option B unless they have a strong reason to deviate — the issue body should be updated with a comment explaining any alternative choice.

Acceptance

• Pages site is reachable at https://lugassawan.github.io/rimba (or a configured custom domain)
• README.md, docs/commands.md, and docs/configuration.md all render without broken links
• Build runs automatically in CI on every push to main
• No docs/ content is lost or broken relative to the current markdown files
• Option B (themed Jekyll with just-the-docs) is implemented, or an alternative is justified in the PR description