docs: add project icon assets (#47)

Adds the PNG and SVG icon for pyswarm alongside the banner, so the raster and
vector marks live in the repo rather than only in the shared assets store.

Nothing references them yet -- wiring one up as the docs favicon or logo is
a separate, per-project decision.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: saudzahirr