docs: add GitHub Pages documentation site

[Fraser-Isbester]

Summary

• Add Hugo + Hextra documentation site under docs/ with getting started, views, rules engine, key bindings, and development pages
• Add gomarkdoc-based API reference generation for all internal packages
• Add GitHub Actions workflow (.github/workflows/docs.yml) to build and deploy docs to GitHub Pages on push to main
• Add package-level doc comments to config, db, rules, tui, theme, and views packages
• Update .gitignore with Hugo build artifacts and generated reference docs

Test Plan

• Verify go build ./..., go vet ./..., and go test ./... pass (confirmed locally)
• Verify gofmt -l . reports no formatting issues (confirmed locally)
• Verify the docs CI workflow runs successfully on this branch
• Verify Hugo site builds with cd docs && hugo --minify after gomarkdoc generation
• Enable GitHub Pages (Settings > Pages > Source: GitHub Actions) and confirm deployment on merge

[Fraser-Isbester]

The GitHub Pages docs site is already live from main (site/ + .github/workflows/pages.yml → https://fraser-isbester.github.io/tusk/), so this Hugo-based site now conflicts with it — two workflows deploying to the same Pages environment. Closing as superseded. Keeping the branch: the package-level doc comments added here (config/db/rules/tui/theme/views) are still worth having, since they'd enrich the generated API reference on the live site. Worth salvaging into a small standalone PR.