docs: restructure manual sidebar to match gh CLI layout

[q-soriarty]

Problem

Our manual sidebar has multiple sections (Getting Started, Command Reference, Guides) with installation, quick start, configuration, and guides visible. The gh CLI manual has a much simpler sidebar structure:

gh CLI sidebar:

• "Getting started" (single page with install + usage)
• Then directly all commands alphabetically (gh, agent-task, alias, api, auth, ...)
• No separate "Command Reference" section header
• No guides section in sidebar

Our sidebar:

• Introduction
• Getting Started (Installation, Quick Start, Configuration)
• Command Reference (all commands)
• Guides (Shell Completion, JSON Output, CI/CD)

Proposed changes

1. Merge Installation, Quick Start, Configuration into a single "Getting started" page
2. Remove "Command Reference" section header — commands listed directly after Getting started
3. Remove Guides from sidebar — link from Getting started or relevant command pages
4. Simplify to match gh CLI: Getting started → commands (flat list)

Reference

https://cli.github.com/manual/

[q-soriarty]

Superseded by #132 (Jekyll documentation migration). Will be closed when #132 is merged.

[github-actions]

Closed by #136 (merged into develop).