docs: follow gin's README layout conventions

Align the structure with gin's README:

- `## Contents` nested TOC (including the title and Contents
  itself), one entry per heading
- Installation first, then a Quick start section with the minimal
  run/init flow
- Features as a checklist
- expand the collapsed <details> Docker examples inline; gin keeps
  everything expanded
- drop bare-link headings in favour of plain heading text with the
  link in the body

All 39 TOC anchors verified against the headings.

Author: xiantang