docs: streamline install section and add community overview (#2561)

- Shorten README.md install section to single uv command + link to
  installation guide for alternatives and troubleshooting
- Add explicit 'Initialize a project' step to README Get Started
- Remove duplicate Troubleshooting section from README
- Reorder 'Make it your own' card on docs landing page so extensions
  and presets are explained before the stats
- Update Community nav-card to link to new community overview
- Create docs/community/overview.md landing page (aligned with
  reference/overview.md)
- Create dedicated install sub-pages: pipx, one-time (uvx), air-gapped
- Update docs/installation.md to lead with persistent uv install and
  link to sub-pages instead of duplicating content
- Update docs/toc.yml with new pages
- Remove stale EOF file