Improve README hero section and first-run path

[httpdss]

Summary

Refresh the README hero section so new visitors immediately understand collectd-web, see the UI, and know how to try it safely.

Why this matters

collectd-web already has meaningful stars/forks, but new visitors need a clearer first-screen path from “what is this?” to “I can install/test this.”

Scope

• Add a concise one-line value proposition near the top.
• Add a prominent screenshot/GIF of the dashboard.
• Clarify project status: stable/maintenance mode plus Python backend rewrite in progress.
• Add a quickstart path for the most common install/test flow.
• Add clear CTAs: install, test the Python backend, open a discussion, contribute.
• Link to install docs, deployment docs, discussions, and contributing guide.

Acceptance criteria

• First screen answers: what it is, who it is for, current status, and how to try it.
• README includes a current screenshot or GIF.
• Python rewrite CTA is visible but does not make the stable project seem unsafe.
• All commands and links are verified.