Docs: replace the Serve and Funnel images with one diagram

[jackspiering]

Description

Replaces the two quickly made Serve and Funnel images in the root README with one SVG diagram and a step-by-step explanation.

• The new diagram shows the real ScaleTail setup: a Docker host with the Tailscale container and the application container in one network namespace.
• Path 1 (Serve, always on): a Tailnet device reaches the Tailscale container over WireGuard, and Serve proxies to the application.
• Path 2 (Funnel, off by default): a visitor on the internet goes through a Funnel relay server, which opens an encrypted TCP proxy to the same Tailscale container.
• The old images implied that Serve and Funnel are separate services and that Funnel gives "filtered access" to one service. Neither matches how the stacks work.
• The text is split into "Serve", "Funnel", and "Turn Funnel on". The facts about relays, TLS, ports, and requirements follow the Funnel documentation.
• The SVG renders on GitHub in light and dark mode, because it has its own background, and it is easy to edit in a diff.
• Removes images/tailscale-serve.png and images/tailscale-funnel.png. Nothing else referenced them.

Related Issues

• None.

Verification

• docker compose config --quiet: not applicable, no service changed
• The stack started: not applicable, documentation only
• The main function of the service works: not applicable, documentation only
• rumdl check --config .markdownlint.yml . on the whole repository (no issues), and git diff --check origin/main
• Rendered the SVG to a PNG with resvg to check the layout. The GitHub rendering was not checked.

Impact on existing users

• None.

Checklist

• I have performed a self-review of my code and followed the templates structure.
• I did not commit a working auth key, password, token, or other credential.
• The service README uses the headings of the template: not applicable, no service changed.
• I added, removed, or renamed the service in the root README.md: not applicable, this PR does not change the list of services.

Additional Context

• None.