feat: migrate documentation from mdBook to Jekyll (gh CLI parity)

[q-soriarty]

Summary

Migrate the Copia CLI manual from mdBook to Jekyll, replicating the gh CLI manual (https://cli.github.com/manual/) as closely as possible.

Spec

docs/superpowers/specs/2026-04-02-jekyll-docs-migration-design.md

Plan

docs/superpowers/plans/2026-04-02-jekyll-docs-migration.md

Consolidates

Closes #125
Closes #126
Closes #127
Closes #128
Closes #129
Closes #130
Closes #83
Closes #77

Phases

1. Generator & Jekyll Foundation — adapt gh CLI internal/docs/markdown.go, create Jekyll site structure
2. Command Content Enrichment — command groups, Long descriptions, example comments
3. Docs & README Migration — reorganize docs/, install guides, README restructure
4. CI/CD & Cleanup — workflow update, mdBook removal, visual verification