docs: rework the README around a 3-step quick start

[loehub]

Pull Request

Summary

Reworks the README so an engineer can scan it the way GitHub READMEs are usually read, and get ros2_medkit running first.

• Top: the selfpatch.ai logo (links to the website), a compact header, and the same badges as on main, unchanged (CI, codecov, docs, license, ROS 2 distros, Discord).
• Short description: what it does in one paragraph, with no code changes needed.
• Quick start in three steps: get Docker, run the existing docker run command (unchanged), then curl /api/v1/apps to see every node it found, which is never empty on a running robot, and /api/v1/faults. A green tip confirms you're done.
• No robot at hand: a pointer to the sensor demo in selfpatch_demos, where ./inject-nan.sh makes a fault appear.
• Extras in accordions: what a fault looks like (JSON, snapshot, rosbag), live stream and web UI, installing without Docker, troubleshooting.
• Standard sections: How it works (one diagram), Features, Compared with standard ROS 2 diagnostics (a markdown table), Ecosystem, Documentation, Contributing, License.

Removed: the Medkit Pro panel, the marketing copy, the image panels and the demo GIF. Copy is in plain language, and no other products are named.

Images: the logo, header and diagram live in selfpatch/.github (merged in selfpatch/.github#4) and load from raw.githubusercontent.com, so this repository carries no image binaries for the README. This PR also removes the unused 15 MB docs/_static/images/medkit_hero.gif.

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Issue

No linked issue. This is a docs-only rework requested by the team; no issue tracks it.

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Type

• Bug fix
• New feature or tests
• Breaking change
• Documentation only

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Testing

• Rendered the README through GitHub's markdown API in a local mockup of the repository page. Checked light and dark themes and opened every accordion. No images are broken.
• All six image URLs on raw.githubusercontent.com/selfpatch/.github/main/profile/assets/ros2_medkit/ return 200 now that Host the ros2_medkit README images next to the profile .github#4 is merged.
• Checked the commands against the code. bringup.launch.py binds 127.0.0.1 by default, so the remote-access tip adds server_host:=0.0.0.0. Its default CORS origins already allow the web UI on localhost:3000. /diagnostics stays opt-in via enable_diagnostic_bridge:=true.
• Checked that the docs links resolve (installation, getting started, troubleshooting, REST API reference, web UI and integration tutorials).
• Nothing else referenced medkit_hero.gif.

Hidden anchors keep the old deep links working: #run-it-in-5-minutes and #vs-standard-ros-2-diagnostics.

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Checklist

• Breaking changes are clearly described (and announced in docs / changelog if needed)
• Tests were added or updated if needed
• Docs were updated if behavior or public API changed