docs: document how to maintain the Asciinema demo (#306)

Author: Djaytan

docs/asciinema/README.md

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+# 🎞️ Asciinema
+
+For full documentation, visit [docs.asciinema.org](https://docs.asciinema.org/).
+
+## ⚙️ Prerequisites
+
+Make sure the following tools are installed:
+
+- 📼 [asciinema CLI](https://asciinema.org/docs/installation) – used to record terminal sessions
+- 🖼️ [agg CLI](https://docs.asciinema.org/manual/agg/installation/) – used to convert recordings into GIFs
+
+## 📝 Format
+
+Recordings use the [Asciicast v2 format](https://docs.asciinema.org/manual/asciicast/v2/).
+
+## Publishing Recording
+
+The recording must be published under [this profile](https://asciinema.org/~Djaytan).
+
+This can be done by running the following commands:
+
+```bash
+$ asciinema auth
+```
+
+##  🧰 Generating a GIF
+
+To generate a GIF from a recording, refer to the [official guide](https://docs.asciinema.org/getting-started/#generating-a-gif).
+
+In short, run:
+
+```bash
+agg demo-gif.cast demo.gif
+```