docs: demo guide (asciinema recording/playback/upload) EN+PT-BR

docs/en/demo.md

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+# Demo (asciinema)
+
+The lab can be recorded as an [asciinema](https://asciinema.org) cast.  Every
+byte in the recording is the real output of the lab VMs (Alpine under libvirt
+running the kernel built from source with `pf.ko`); only the pacing is
+scripted.
+
+## Record
+
+```sh
+pip install asciinema            # once
+bash scripts/vm-run-tests.sh      # or: scripts/vm-create.sh + deploy
+asciinema rec out/demo-pf-linux.cast -c "bash scripts/demo-session.sh"
+```
+
+The session walks through:
+
+1. the lab topology and the freshly loaded `pf.ko` (`dmesg` proof),
+2. `pfctl -s info` over the OpenBSD ioctl ABI,
+3. a real `pf.conf` (table, stateful rules, explicit drop) and `pfctl -sr`,
+4. transit ping + `pfctl -ss` (stateful),
+5. `pfctl -t <table> -T show` (live tables),
+6. a blocked ping (counters with `pfctl -sr -vv`),
+7. `divert-to` with the `pdivert` relay (packet to userspace and back),
+8. shutdown.
+
+## Play back
+
+```sh
+asciinema play out/demo-pf-linux.cast
+```
+
+## Upload
+
+```sh
+asciinema upload out/demo-pf-linux.cast
+```
+
+`scripts/demo-record.py` is an alternative recorder that produces the same
+asciicast v2 format without the asciinema binary (it drives the lab over
+ssh and writes the cast itself).