🎨 assets: add a logo, one plotting call dispatched to three backends

[nstarman]

plotting_backends had no logo. This gives it one: a dot, the plotting call, branching to three small charts. Each chart shows the same plot, drawn by a different backend in its own colour (teal, blue and purple), in GalacticDynamics' navy.

docs/_static/make_logo.py writes it:

uv run docs/_static/make_logo.py                     # docs/_static/favicon.svg
uv run docs/_static/make_logo.py --size 2048 big.png # a PNG, at any size

• SVG by default: 1 KB and sharp at any size. The script writes the SVG text directly, so the default needs no dependencies.
• Geometry: the backends' colours, the call, the charts' placement and the plot they share are constants at the top of the script. Adding a backend is one more colour and one more chart position.
• PNG on request: rendered with resvg-py, declared in the script's PEP 723 header, so uv run fetches it.
• Location: docs/_static/, as in the other GalacticDynamics packages, ready for when plotting_backends has docs. Nothing references it yet.

Checked:

• The pre-commit hooks pass on the new files. The mypy hook is limited to src/ and tests/, so it doesn't apply.
• Re-running the script reproduces the file, and it stays readable at 32 px.

Part of giving the GalacticDynamics packages scripted logos.

🤖 Generated with Claude Code