docs: migrate README content to the Read the Docs site

[shejnowicz]

Follow-up to #110 (scaffold). Move the remaining README sections into docs/ pages and shrink the README to a landing page:

• Getting started: full install options (Docker image / installer / git clone), devcontainer flow, IDE specifics (VS Code, JetBrains)
• Configuration: settings format & layers, applying changes, secret substitution (op://, pass://, plain values), extra_hosts, upstream CA bundles
• CLI reference: fold in cli/README.md (init/run/compose/edit/cache/restart/destroy/proxy/attach/version)
• Security: hardening VS Code, volume split (security: sharing ~/.mitmproxy to app #25), no-new-privileges/sudo removal, strict-network once feat(cli): --features strict-network — stack presets instead of the wildcard #106 lands
• Operations: testing the proxy, debugging, unit tests
• Project: inspiration, development, commercial support

Rules of thumb: pages lifted verbatim where possible (heading levels normalized), internal anchors updated as sections move, README ends as a short landing page linking to the site.