docs: reorganize README into a docs/ guide with a table of contents

[reckerp]

Summary

The README had grown into a single ~530-line page covering installation, every command, and all configuration. This PR splits it into a focused landing page plus a structured docs/ folder with a table of contents and one page per topic — easier to scan, navigate, and maintain.

Created in a fresh workspace via gx workspace new docs/restructure (dogfooding).

What changed

README.md → slim landing page (528 → 99 lines)

• Highlights, installation, a runnable Quick start, the command table, a Documentation section linking into docs/, development, and license.

New docs/ folder (9 pages, ~880 lines)

• docs/README.md — documentation index / table of contents + a per-command reference table.
• getting-started.md — prerequisites, install, shell integration, and a first end-to-end workflow.
• commands.md — checkout, status, add, commit, push, stash, log, and git pass-through.
• workspaces.md — worktree concepts, create/switch/update/remove, setup vs sync, utility commands, the cleanup lifecycle + safety, the TUI, and cd-on-switch.
• pull-requests.md — the PR dashboard.
• onboarding.md — shared vs. personal config, the .gx/workspace.toml schema, hooks, and layering.
• shell-integration.md — gx setup.
• configuration.md — the config.toml reference.
• skills.md — agent skills.

Notes

• No documented behavior changed. Existing content was reorganized and lightly expanded (concept intros, a Getting Started workflow, cross-links, and per-page prev/next navigation). No new features were invented.
• Each page links back to the index and to the next topic.
• All internal links and anchors were validated with a script (10 files, every relative link + heading anchor resolves).

Testing

Docs-only change; no source touched.