docs: add a terminal demo GIF to the README (#312)

Closes #309.

Adds `docs/assets/podup-demo.gif` — a clean GitHub-dark terminal demo of
a **real** `podup up -d` / `podup ps` session against rootless Podman
5.4.2 (command output is verbatim; only the typing animation is
synthetic). Embedded as the README visual hook above the architecture
Mermaid. In-repo asset, no external hotlink (per the docs standard). 57
KB, 860×292.

This was the last open item — the visual hook deferred from #294.
`Closes #N` inert on squash→develop; close manually.

Signed-off-by: Jaro-c <75870284+Jaro-c@users.noreply.github.com>

Author: Jaro-c

README.md

@@ -6,6 +6,10 @@
 **podup** runs your `docker-compose.yml` on rootless Podman — a single static
 binary, written in Rust, with no daemon and no Python runtime.
 
+<p align="center">
+  <img src="docs/assets/podup-demo.gif" alt="podup up and podup ps running a compose stack on rootless Podman" width="760">
+</p>
+
 ```mermaid
 flowchart LR
 	A["docker-compose.yml"] --> B["podup"]